Lie to the Swift compiler about isolation

[Supereg]

Lie to the Swift compiler about isolation

♻️ Current situation & Problem

In PR #14 we truthfully implemented strict concurrency checking for the XCTRuntimeAssertions library. However, this made it impossible to test runtime precondition failures on the @MainActor as it would completely block the actor (or the Task would only be scheduled after the XCTRuntimePrecondition call returned).
Therefore, it is necessary to always run XCTRuntimePrecondition expressions on a background thread. If we would keep correct @Sendable annotations for the expression closure, this would make it impossible to run XCTRuntimePreconditions on the @MainActor. Therefore, we lie to the compiler. We deliberately don't annotate the expression closure as @Sendable and silence the respective concurrency warning.
The impact of that is documented.

⚙️ Release Notes

• Restore testing preconditions which are annotated to be isolated to the @MainActor.

📚 Documentation

Add documentation bubbles to explain the impact.

✅ Testing

Add unit testing to prevent regressions.

Code of Conduct & Contributing Guidelines

By submitting creating this pull request, you agree to follow our Code of Conduct and Contributing Guidelines:

• I agree to follow the Code of Conduct and Contributing Guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.48%. Comparing base (2ebbbb4) to head (d00ef0f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   91.34%   91.48%   +0.15%     
==========================================
  Files           8        8              
  Lines         300      305       +5     
==========================================
+ Hits          274      279       +5     
  Misses         26       26              

Files	Coverage Δ
.../XCTRuntimeAssertions/XCTRuntimePrecondition.swift	98.08% <100.00%> (+0.10%)	⬆️

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