Adjust a timing-sensitive TimeLimitTrait test to avoid flakiness in CI

[stmontgomery]

This adjusts the TimeLimitTraitTests.cancelledTestExitsEarly() test function to make it less sensitive to running on resource-constrained CI systems.

Motivation:

Recently on a few PRs I have seen flaky results from this test which are unrelated to my work. They typically show up as console output such as:

✘ Test "Cancelled tests can exit early (cancellation checking works)" recorded an issue
  at TimeLimitTraitTests.swift:193:5: Expectation failed:
  (timeAwaited → 5.108262525 seconds) < (.seconds(5) → 5.0 seconds)

This indicates the test is just a little too sensitive to specific timing.

Modifications:

• Lengthen the "full" sleep duration the test would run if it wasn't cancelled, which it is.
• Extend the much shorter duration it allows for waiting, to avoid false positives when running on resource-constrained CI systems.

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