fix fixtures not loading if a belongs_to association is defined with a :foreign_key option that's a symbol

Restore the 4.0 behaviour for 'sqlite3:///', but deprecate it. We'll
change to the absolute-path interpretation in 4.2.

The current "correct" spellings for in-memory, relative, and absolute
URLs, respectively, are:

    sqlite3::memory:
    sqlite3:relative/path
    sqlite3:/full/path

Substantially reverses/defers fbb79b51.
Uncovered by @guilleiguaran while investigating #14495, though that
sounds like a different issue.

Allows fixtures to use their $LABEL as part of a string instead
of limiting use to the entire value.

    mark:
      first_name: $LABEL
      username: $LABEL1973
      email: $LABEL@$LABELmail.com

    users(:mark).first_name # => mark
    users(:mark).username   # => mark1973
    users(:mark).email      # => mark@markmail.com

Fixes: #11524

Fix bad test order dependency between LoadAllFixturesTest#test_all_there and FoxyFixturesTest#test_ignores_belongs_to_symbols_if_association_and_foreign_key_are_named_the_same

Stubbing ENV[] is not safe outside MRI. At some point after the
stubbing has occurred a backtrace is printed to the ActiveSupport
warning log: there Rubinius accesses ENV['RBX_NOCOLOR'] to determine
if it should print the backtrace with colors or not, causing the
stub to fail. Other implementations might access ENV in a different
way too, we just can't predict it.
The only thing we can do here is to actually set the ENV with what
we want and restore it afterwards.

Allows you to change your configuration for calls to
`table_name_prefix`, `table_name_suffix`, and `pluralize_table_names`.
The default configuration is still ActiveRecord::Base, but you are now
able to change the configuration easily.

Also, constantizing the default_fixture_model_name when it gets loaded
in from the file. Later, when the class_name is passed to a new
FixtureSet, a deprecation warning will occur if the class_name is a
string.

As you can also configure your database connection using `ENV["DATABASE_URL"]`,
the fixture setup can't reply on the `.configurations` Hash.

As the fixtures are only loaded when ActiveRecord is actually used
(`rails/test_help.rb`) it should be safe to drop the check for an existing configuration.

This allows end-users to have a `connection` method on their models
without clashing with ActiveRecord internals.

This reverts commit 637a7d9d, reversing
changes made to 5937bd02.

Rename `ActiveRecord::Fixtures` class to `ActiveRecord::FixtureSet`.  Instances of this class normally hold a collection of fixtures (records) loaded either from a single YAML file, or from a file and a folder with the same name.  This change make the class name singular and makes the class easier to distinguish from the modules like `ActiveRecord::TestFixtures`, which operates on multiple fixture sets, or `DelegatingFixtures`, `::Fixtures`, etc., and from the class `ActiveRecord::Fixture`, which corresponds to a single fixture.

This reverts commit 7381596e.

create_fixtures should accept symbol keys for custom table -> class mappings. Fixes an unintended regression.

setup_fixture_accessors should accept symbols for table names. Fixes an unintended (and untested) regression.

Test using fixtures with random names and model names, that is not following naming conventions but using set_fixture_class instead.

It is expected that the table name be defined in the model, but this is not explicitly tested here.  This will need to be fixed.

Make sure the table name of a model is reset in a test case after assigning ActiveRecord::Base.table_name_prefix and ActiveRecord::Base.table_name_suffix.

This reverts commit f8e484d0, reversing
changes made to fa5adfb1.

Reason: broke the postgres tests.

Make sure the table name of a model is reset in a test case after assigning ActiveRecord::Base.table_name_prefix and ActiveRecord::Base.table_name_suffix.  This was somebody else's test case, so an independent opinion on the change can be helpful.