*testing.T with context.Context and middleware

Package testctx aims to represent what *testing.T might have looked like if context.Context existed at the time it was created.

The main benefit of joining these two packages is support for test middleware, which can greatly cut down on repetition (t.Parallel()) and allow for transparent test instrumentation.

Go 1.24 adds a t.Context() accessor which provides a context.Context that will be canceled when the test completes, just before t.Cleanup-registered funcs are executed. However, it does not provide any way to modify the context.

By contrast, this package provides both a t.Context() and a t.WithContext(ctx) for modifying the context that is passed to sub-tests, and additionally passes the t.Context() value as a ctx argument to tests.