Change the default profile on tests from Standard to Testing

Description of the need

Currently most tests are using the Standard profile. This causes lots of unnecessary overhead on each test because most tests only need a few modules installed.

Proposed solution

Change the default profile to one that has fewer tables to copy when setting up a new test prefix.

Additional information

This will probably break tests in contrib. This should be easy to mitigate by setting protected $profile = 'standard'; if you depend on something from the standard profile, but it will require changes from module maintainers.