test/librados/aio_cxx: Fix potential mutex destruction failure on PoolEIOFlag

test/librados/aio_cxx: Fix potential mutex destruction failure on PoolEIOFlag

GTEST_SKIP() does not return and causes early test exit, which
skips any following cleanup logic.
In this case, the test skipped before releasing my_lock, causing
pthread_mutex_destroy() to fail with EBUSY and trigger a ceph_assert()
in the mutex debug wrapper.
Fix by explicitly unlocking the mutex before calling GTEST_SKIP().

Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>