Enable Test.cancel() as API.

[grynspan]

Pending approval of ST-0016.

Checklist:

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

[grynspan]

@iamleeg Would you please have a look at the documentation changes here? Thanks!

[iamleeg]

I think present tense is better throughout to make the prose more engaging, for example:

If a test is running and you determine it cannot complete and
needs to end early without failing, use Test/cancel(_:sourceLocation:) to
cancel the test:

Other than that, this all looks great, thanks!