cmd/go/internal/lockedfile: avoid failing tests due to arbitrary timeouts
The mustBlock helper returns a function that verifies that the blocked
operation does eventually complete. However, it used an arbitrary
10-second timeout in place of “eventually”.
Since the test is checking a synchronization library, bugs are likely
to manifest as deadlocks. It may be useful to log what operation is in
flight if such a deadlock occurs; however, since we can't bound how
long a “reasonable” operation should take, the log message should only
be informational — it should not cause the test to fail.
While we're here, let's also set a better example by not leaking
time.After timers in the tests..