-   Added run_cmd() class method to dry up Kernel.system() messages within
      this namespace and avoid shell expansion by passing a list of
      arguments instead of a string

  -   Update structure_dump, structure_load, and related tests units to
      pass a list of params instead of using a string to
      avoid shell expansion

This isn't Seattle.rb, @senny. ;)

Closes #17945

`db:test:prepare` still purges the database to always keep the test
database in a consistent state.

This patch introduces new problems with `db:schema:load`. Prior
to the introduction of foreign-keys, we could run this file against
a non-empty database. Since every `create_table` containted the
`force: true` option, this would recreate tables when loading the schema.

However with foreign-keys in place, `force: true` wont work anymore and
the task will crash.

/cc @schneems

The current style for warning messages without newlines uses
concatenation of string literals with manual trailing spaces
where needed.

Heredocs have better readability, and with `squish` we can still
produce a single line.

This is a similar use case to the one that motivated defining
`strip_heredoc`, heredocs are super clean.

[Joshua Cody & Yves Senn]

Closes #16757.

Prior to this patch schema loading rake tasks had the potential to leak a
connection to a different database. This had side-effects when rake tasks
operating on the current connection (like `db:seed`) were chained.

This fixes random test failure in migrator_test.rb

Using heredoc would enforce line wrapping to whatever column width we decided to
use in the code, making it difficult for the users to read on some consoles.

This does make the source code read slightly worse and a bit more error-prone,
but this seems like a fair price to pay since the primary purpose for these
messages are for the users to read and the code will not stick around for too
long.

The rake tasks and the `DatabaseTakss` adapter classes used to
assume a configuration at some places. This forced the rake
tasks to establish a specific connection before calling into
`load_schema`.

After #15394 this started to cause issues because it could
`purge` the wrong database before loading the schema.

Revert "Revert "Merge pull request #15394 from morgoth/fix-automatic-maintaining-test-schema-for-sql-format""

This reverts commit 5c87b5c5.

This extracts the logic that was embedded in a Rake task into a static
method.
Bonus: the first test for `rake db:migrate`

Revert "Merge pull request #15394 from morgoth/fix-automatic-maintaining-test-schema-for-sql-format"

This reverts commit 46139d33, reversing
changes made to 8f247871.

Conflicts:
	activerecord/CHANGELOG.md

Additionally:
* It changes `purge` task on `sqlite3` adapter to recreate database file, to
be consistent with other adapters.
* Adds `purge` step when loading from `schema.rb`

rather than YAML.load(File.read(path)). YAML.load_file is also used in
guides/rails_guides/helper.rb since 2011, the only other precedent I could find.

Closes #13625

* Move check from generated helper to test_help.rb, so that all
  applications can benefit
* Rather than just raising when the test schema has pending migrations,
  try to load in the schema and only raise if there are pending
  migrations afterwards
* Opt out of the check by setting
  config.active_record.maintain_test_schema = false
* Deprecate db:test:* tasks. The test helper is now fully responsible
  for maintaining the test schema, so we don't need rake tasks for this.
  This is also a speed improvement since we're no longer reloading the
  test database on every call to "rake test".

Warning:

  DEPRECATION WARNING: Passing a string to ActiveRecord::Base.establish_connection for a configuration lookup is deprecated, please pass a symbol (:development) instead.

This commit also cleans up the rake tasks that were checking
for DATABASE_URL in different places.

In fact, it would be nice to deprecate DATABASE_URL usage in the long
term, considering the direction we are moving of allowing those in .yml
files.

This reverts commit 22f80ae5.

See
https://github.com/rails/rails/commit/22f80ae57b26907f662b7fd50a7270a6381e527e#commitcomment-4640676

Conflicts:
	activerecord/CHANGELOG.md

* A non-zero exit status allows subsequent shell commands to be chained
  together such as: `rake db:reset test:prepare && rspec && cap deploy`
  (if you're feeling brave :)
* Any exceptions raised during the `create` and `drop` tasks are caught
  in order to print a "pretty" message to the user. Unfortunately doing
  so prevents rake from aborting with a non-zero exit status to the shell.
* Therefore we re-raise the exceptions after the "pretty" message and
  re-catch them in the task.
* From the task we explicitly exit with a non-zero status. This method
  was chosen (rather than just letting rake fail from the exception) so
  that the backtrace is suppressed and the output to stderr is
  unchanged.
* Update activerecord CHANGELOG

Same as 4d4ff531

It's simpler to assume that passed env is just a string without any
extensions, especially when DatabaseTasks are intended to be used also
without rails.

This is a public class, which could be used directly in order to allow
easier database management. Now it also contains settings, which will be
used by databases.rake.

This will allow to set fixtures path in global config easier, it should
be not needed to use FIXTURES_PATH every time user wants to load
fixtures.