[libcxx testing] Remove ALLOW_RETRIES from lock_guard tests

[libcxx testing] Remove ALLOW_RETRIES from lock_guard tests

These two tests were clumsily using time measurements to determine
whether std::lock_guard was working correctly. In practice, this
approach merely verified that the underlying lock properly waits.

Now these two tests verify that lock is acquired, not dropped
prematurely, and finally, actually dropped at the end of the scope.