unittest_lockdep: avoid any threads for death test

unittest_lockdep: avoid any threads for death test

Heed the warning:

[WARNING] /home/sage/src/ceph/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 4 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.

Fixes: https://tracker.ceph.com/issues/43403
Signed-off-by: Sage Weil <sage@redhat.com>