test: do not consider warning from Sanitizer as fatal

test: do not consider warning from Sanitizer as fatal

with sanitizer enabled, unittest_ceph_crypto fails like

```
[ RUN      ] ForkDeathTest.MD5

[WARNING] /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest-death-test.cc:1121:: Death tests use fork(), which is unsafe particularly in a threaded context. For this test, Google Test detected 3 threads. See https://github.com/google/googletest/blob/master/googletest/docs/advanced.md#death-tests-and-threads for more explanation and suggested solutions, especially if this is the last message you see before your test times out.
/home/jenkins-build/build/workspace/ceph-pull-requests/src/test/ceph_crypto.cc:273: Failure
Death test: do_simple_crypto()
    Result: died but not with expected error.
  Expected: contains regular expression "^$"
Actual msg:
[  DEATH   ] ==3798016==Running thread 3797882 was not suspended. False leaks are possible.
[  DEATH   ] ==3798016==Running thread 3797885 was not suspended. False leaks are possible.
[  DEATH   ]
[  FAILED  ] ForkDeathTest.MD5 (119 ms)
```

but this error message should not considered as an indication of
fatal error. so, in this change, instead of matching the output with
a regex of "^$", we use a matcher to match with the error message if
sanitizer is enabled.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>