Introduce 'MarkerTrait' to unify the representation of boolean test attributes

[stmontgomery]

This introduces a utility type named MarkerTrait which acts as a generic "marker" trait representing a boolean attribute of a test, and uses it to replace the .hidden trait and Test.isSynthesized property.

Motivation:

Currently the testing library has an internal-only trait named .hidden, of type HiddenTrait, which is used in the project's own unit tests to represent fixture/example tests and suites. Separately, it also has a stored, boolean property on Test named isSynthesized which is meant for debugging purposes to keep track of suites which were synthesized during planning and didn't have a @Suite attribute.

Conceptually, both of these can be thought of as boolean attributes of a test/suite. A given Test instance is either hidden or not, and either synthesized or not. There's an opportunity to simplify the modeling by representing them both as a trait, which would make adding future boolean test attributes more straightforward. Plus, for isSynthesized specifically, doing this would eliminate a stored property on every Test instance.

Modifications:

• Add MarkerTrait type and use it reimplement .hidden.
• Remove the Test.isSynthesized stored property and replace it with a new .synthesized marker trait.
• Only include the synthesized trait in debug builds (guarded by #if DEBUG) to avoid needless overhead in release builds where this information isn't used.
• Add a Test.containsTrait() utility function for checking whether a test contains some (equatable) trait.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[grynspan]

I see where you're going with this, but it may make it harder to support Embedded Swift by relying even more on existentials.

There is also a performance cost to this change that we should be careful to measure.