tools: add a test wrapper that replays crashing tests under gdb

meson test --wrapper hook to print a gdb backtrace inline in the test
log when a test exits with an actual crash signal (SIGSEGV, SIGABRT,
SIGBUS, SIGFPE, SIGILL). Wired into the default add_test_setup() so it
runs automatically on every `meson test`.

Environmental terminations (SIGTERM/SIGKILL/SIGPIPE/SIGALRM) are passed
through without replay, and the original signal is re-raised so the
parent's wait() observes WIFSIGNALED rather than a plain exit code.

meson.build

@@ -16,6 +16,7 @@ project('systemd', 'c',
 add_test_setup(
         'default',
         exclude_suites : ['clang-tidy', 'unused-symbols', 'integration-tests'],
+        exe_wrapper : find_program('tools/test-crash-trace.sh'),
         is_default : true,
 )