test/ceph_assert: fix death test timeout with threadsafe style

test/ceph_assert: fix death test timeout with threadsafe style

The unittest_ceph_assert test was timing out intermittently with warning:
```
"Death tests use fork(), which is unsafe particularly in a threaded
context. For this test, Google Test detected 3 threads."
```
The issue occurs because global_init() creates threads before the death
tests run. Use the "threadsafe" death test style to handle this safely,
matching the pattern already used in ceph_breakpad.cc and
test_perf_counters_cache.cc.

see also https://github.com/google/googletest/blob/main/docs/advanced.md#death-tests-and-threads

Signed-off-by: Kefu Chai <k.chai@proxmox.com>