Add VerifyCleanup to support leak detection with t.Cleanup

[AdamMagued]

Summary

Adds VerifyCleanup(t TestingTB, options ...Option) to enable goroutine leak detection using testing.TB.Cleanup.

Fixes #53

Problem & Motivation

The standard recommendation for per-test leak detection is defer goleak.VerifyNone(t). However, when test setup is abstracted into helper functions, using defer goleak.VerifyNone(t) inside the helper causes verification to run immediately when the helper returns, rather than at the end of the test. As a result, goroutines spawned during the main test execution are never checked.

In addition, callers who previously attempted to wrap VerifyNone in t.Cleanup with options like IgnoreCurrent() would capture goroutines at cleanup time rather than test setup time.

Solution

1. Define TestingTB as the minimal interface required for cleanup-based verification (TestingT + Cleanup(func())).
2. Add VerifyCleanup(t TestingTB, options ...Option):
   • Evaluates options (such as IgnoreCurrent()) immediately at call time to establish the baseline state.
   • Registers a cleanup function via t.Cleanup that invokes VerifyNone(t, opts) when the test finishes.
   • Marks the helper frame via t.Helper() so failures report the test call site.
3. Fix test synchronization in utils_test.go and options_test.go to ensure background test goroutines are cleanly joined across test iterations.

Verification

All existing and newly added tests pass with 100% statement coverage:

• Unit tests verifying VerifyCleanup detects leaks, passes on clean runs, works across test helper boundaries, and correctly captures IgnoreCurrent() at setup time.
• go test -v -race ./... (100% pass)
• go test -race -count=5 ./... (100% pass)
• go test -cover ./... (100.0% coverage across all packages)

[CLAassistant]

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