test(storage): add test for Writer race (#6862)
test(storage): add test for Writer race (#6862)

Adds a test to detect a potential race condition that can arise
if the context is cancelled before writing to GCS. We already
have a test that detects a similar condition after we have
already started writing the object, but this test looks at the context
being cancelled before any writes occur.

When run with -race, this test fails in storage/v1.27.0, but it
passes when run with the fix in #6817.

Updates #6816

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go