Add UnitIO monad and thread-safe golden testing via MonadBaseControl

[newhoggy]

This PR introduces two key features aimed at improving the expressiveness and safety of unit testing in the hedgehog-extras library:

New UnitIO Monad

A new UnitIO monad has been added under Hedgehog.Extras.Test.Unit. This is a wrapper around TestT (ResourceT IO) with support for:

MonadTest, MonadBaseControl IO, MonadIO, MonadResource, MonadCatch, and more.

Integration with Tasty via the Tasty type class.

Convenient usage in unit tests that require resource management and assertions.

A corresponding UnitSpec demonstrates example usage with bracket_ semantics.

Thread-Safe Golden File Support

Golden file operations (diffVsGoldenFile, diffFileVsGoldenFile) are now thread-safe:

A semaphore per canonicalised path is used to protect golden tests from concurrently writing to the same golden files. This allows multiple tests to share the same golden file.

New golden test function

New diffVsGoldenFileExcludeTrace function which excludes stack traces when comparing golden output, improving the stability of golden tests for error messages with non-deterministic trace content.

Miscellaneous

Re-exported the Golden module from Hedgehog.Extras.Test for easier access

This change lays the foundation for more robust and expressive unit testing, while resolving race conditions in golden file testing.

[carbolymer]

Good progress! I'm worried about copying code, which will be annoying to maintain in the future.

Comments addressed