Omit `rails activestorage:install` for jdbcmysql, jdbc and shebang tests

AppGeneratorTest#test_config_jdbcmysql_database

  rails aborted!
  LoadError: Could not load 'active_record/connection_adapters/mysql_adapter'.
  Make sure that the adapter in config/database.yml is valid.
  If you use an adapter other than 'mysql2', 'postgresql' or 'sqlite3' add
  the necessary adapter gem to the Gemfile.
  (compressed)
  bin/rails:4:in `<main>'
  Tasks: TOP => activestorage:install => environment
  (See full trace by running task with --trace)

AppGeneratorTest#test_config_jdbc_database

  rails aborted!
  LoadError: Could not load 'active_record/connection_adapters/jdbc_adapter'.
  Make sure that the adapter in config/database.yml is valid.
  If you use an adapter other than 'mysql2', 'postgresql' or 'sqlite3' add
  the necessary adapter gem to the Gemfile.
  (compressed)
  bin/rails:4:in `<main>'
  Tasks: TOP => activestorage:install => environment
  (See full trace by running task with --trace)

AppGeneratorTest#test_shebang_is_added_to_rails_file

  /home/ubuntu/.rbenv/versions/2.4.1/bin/ruby: no Ruby script found in input (LoadError)

Prevent PendingMigrationError in tests

 * Run `bin/rails db:migrate RAILS_ENV=test` in test_cases before start tests to prevent PendingMigrationError
 * FileUtils.rm_r("db/migrate")
 * --skip-active-storage

Fix failed tests in `railties/test/railties/engine_test.rb`

Related to #30111

Imporve `SharedGeneratorTests#test_default_frameworks_are_required_when_others_are_removed`

 - Explicitly skip active_storage
 - Ensure that skipped frameworks are commented
 - Ensure that default frameworks are not commented

Fix error `Errno::ENOSPC: No space left on device - sendfile`

Since `rails new` runs `rails active_storage:install`
that boots an app.

Since adding Bootsnap 0312a5c6
during booting an app, it creates the cache:

   264K    tmp/cache/bootsnap-load-path-cache
   27M     tmp/cache/bootsnap-compile-cache

* teardown_app must remove app

This reverts commit 3420a145, reversing
changes made to afb66a5a.

".. with __dir__ we can restore order in the Universe." - by @fxn

Related to 5b8738c2

Make sure the file doesn't exist until we've finished writing it.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

* This allows for easier reading, since those are two words, so they should be
  split by _
Signed-off-by: NJeroen van Baarsen <jeroenvanbaarsen@gmail.com>

ref: https://github.com/rails/rails/pull/18763#issuecomment-72349769

In passing, avoid a blind retry in QC: instead, just fix the problem.