Highlight parallel-by-default test run in case of XCTest migration

[petermolnar-dev]

Adding highlight and link to the XCTest Migration document to help understand and mitigate the parallel-by-default test execution.

Motivation:

Handling #863

Modifications:

Adding the Important aside, highlighting the parallel-by-default nature of Swift Testing test execution while migrating from XCTest.

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 help us review?

[grynspan]

@petermolnar-dev It looks like you've opened this PR and made changes using two separate GitHub accounts. Was this intentional?

[iamleeg]

Thanks for making this PR!

[iamleeg]

This sentence doesn't say anything that isn't already covered in "Run tests sequentially," below. Additionally, the aside's content isn't really relevant to the topic of this section, which is about creating test suites. I'd recommend thinking about the structure of this content differently.

Is there something that "Run tests sequentially" ought to tell developers, but doesn't? If so, add information there.

Is there something that "Convert test classes" ought to tell developers about concurrency, so they know to read "Run tests sequentially"? If so, work that into the prose in this section.

If your goal is to ensure that developers remember to think about concurrency, they should probably also remember to think about some of the other points in this doc. Perhaps you need to expand the Overview, to give developers a list of important points to consider when they migrate tests.

[stmontgomery]

Hi @petermolnar-dev, thank you for the PR. It looks like your motivation for submitting it was to resolve the issue #863, is that correct? We realized upon reviewing that issue that it was probably resolved already by @iamleeg's subsequent PR #960 — or at least, it's not clear what additional information beyond that should be added — we just forgot to close #863 after #960 was merged.

I'm going to close this PR for now given that. However, if you feel there is additional guidance on the Migrating document which should still be added, please clarify and we can re-open and reconsider this PR.