fix: crash when trace-logging in tests (#5529)

This PR fixes a crash in tests when the test `Env is run at trace/debug log level.

This issue only affects tests, and only if logging at trace/debug level, so really only relevant during rippled development, and does not affect production servers.

src/test/unit_test/SuiteJournal.h

@@ -94,6 +94,8 @@ SuiteJournalSink::writeAlways(
         return "FTL:";
     }();
 
+    static std::mutex log_mutex;
+    std::lock_guard lock(log_mutex);
     suite_.log << s << partition_ << text << std::endl;
 }