[SWT-NNNN] Range-based confirmations

[grynspan]

Swift Testing includes an interface for checking that some asynchronous event occurs a given number of times (typically exactly once or never at all.) This proposal enhances that interface to allow arbitrary ranges of event counts so that a test can be written against code that may not always fire said event the exact same number of times.

Read the full proposal here.

Checklist:

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

[grynspan]

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[grynspan]

Stuart has approved this PR and will be updating the forum thread shortly. Merging.

[grynspan]

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