Avoid more test skipping for situations that are actually errors or failures

[johnbillion]

This is a follow-up to #40533 for the 4.9 cycle.

Some tests contain calls to markTestSkipped() as a result of situations such as incorrect test data or unsuitable test environments. If the test data isn't valid or the environment is not capable of supporting the test then the test should fail, otherwise the tests are not reliable.

[johnbillion]

In 41367:

Build/Test Tools: Convert more test skipping into hard failures.

See #41851

[boonebgorges]

@johnbillion The changes to Tests_Image_Meta seem too strong to me. WordPress can run in environments without the gd or exif extensions. If we want the tests to be portable - especially for cases where hosts want to run them in their own environments - then we should not make them fail because of non-essential PHP modules. (That's what phpunit --strict is for.)

If, on the other hand, we're going to decide that we want the tests to be maximally broad, and thus to have system requirements more stringent than WordPress itself, we should establish this as policy.

[johnbillion]

@boonebgorges Agreed on the gd change. Thanks.

[johnbillion]

In 41647:

Build/Test tools: Partially revert [41367] as gd and exif aren't actually required to run WordPress.

See #41851