fuzz: unify logging setup

Make sure we don't log anything when running in "fuzzing" mode. Also,
when at it, unify the setup logic into a helper, pretty similar to
the test_setup_logging() one.

Addresses:
  - https://github.com/systemd/systemd/pull/29558#pullrequestreview-1676060607
  - https://github.com/systemd/systemd/pull/29558#discussion_r1358940663

src/systemctl/fuzz-systemctl-parse-argv.c

@@ -22,10 +22,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         if (size > 16*1024)
                 return 0; /* See the comment below about the limit for strv_length(). */
 
-        /* We don't want to fill the logs with messages about parse errors.
-         * Disable most logging if not running standalone */
-        if (!getenv("SYSTEMD_LOG_LEVEL"))
-                log_set_max_level(LOG_CRIT);
+        fuzz_setup_logging();
 
         arg_pager_flags = PAGER_DISABLE; /* We shouldn't execute the pager */