test: use death_test_style="threadsafe" for Mutex.NormalAsserts

test: use death_test_style="threadsafe" for Mutex.NormalAsserts

use threadsafe for running the Mutex assert test to run the test from
beginning to avoid the hang. this test overrides the symbol of
ceph::__ceph_assert_fail() with a local one offered by the application
links against libceph-common. but we intentionally forbid this behavior:
we do not allow libceph-common to reference the symbols exposed from
application with the same name. see http://tracker.ceph.com/issues/25154

Signed-off-by: Kefu Chai <kchai@redhat.com>