test/objectstore: set "threadsafe" flag for ASSERT_DEATH tests

test/objectstore: set "threadsafe" flag for ASSERT_DEATH tests

we are seeing failures like

192/192 Test #144: unittest_chain_xattr ....................***Timeout 3600.01 sec
[==========] Running 5 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 5 tests from chain_xattr
[ RUN      ] chain_xattr.get_and_set

[WARNING] /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest-death-test.cc:1122:: 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.

so in this change, the "threadsafe" death test style is used for
testing the expected assertion failures.

see also
https://github.com/google/googletest/blob/master/googletest/docs/advanced.md#death-test-styles

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