Fix setup of adding _flash_types test.

Adding flash types to a controller within any of the tests will result
in a global state change of the controller under test.

This patch will prevent state leaks and allow us to run the test in random order.
Merge pull request #14614 from tgxworld/fix_add_flash_types_test

Fix setup of adding _flash_types test.
Added OS X specific commands to installation guide [ci skip]
Recommended using homebrew for installing MySQL and PostgreSQL
fix CollectionProxy delete_all documentation

This method no longer returns an array of all records that
have been removed. Correct documentation to reflect this change.

See issue 14546
Merge pull request #14626 from eileencodes/fix_incorrect_collection_proxy_delete_all_documentation

fix CollectionProxy delete_all documentation
Remove unused `subclass_controller_with_flash_type_bar` var from flash test.
Merge pull request #14624 from jamox/dev_install_guide

Added OS X specific commands to installation guide [ci skip]
Merge pull request #14628 from vipulnsward/remove-flash-test-warning

Remove unused `subclass_controller_with_flash_type_bar` var from flash test.