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/nspawn/fuzz-nspawn-oci.c

@@ -15,10 +15,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         f = data_to_file(data, size);
         assert_se(f);
 
-        /* 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();
 
         (void) oci_load(f, "/dev/null", &s);
 

src/nspawn/fuzz-nspawn-settings.c

@@ -15,10 +15,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         f = data_to_file(data, size);
         assert_se(f);
 
-        /* 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();
 
         (void) settings_load(f, "/dev/null", &s);