fix(tests): prevent buffer corruption from concurrent log writes (#565)

std::endl triggers flush() which calls sync() on the shared log buffer.
Multiple threads racing in sync() cause str()/str("") operations to
corrupt buffer state, leading to crashes and double frees.

Added mutex to serialize access to suite.log, preventing concurrent
sync() calls on the same buffer.

src/test/unit_test/SuiteJournal.h

@@ -19,9 +19,9 @@
 
 #ifndef TEST_UNIT_TEST_SUITE_JOURNAL_H
 #define TEST_UNIT_TEST_SUITE_JOURNAL_H
-
 #include <ripple/beast/unit_test.h>
 #include <ripple/beast/utility/Journal.h>
+#include <mutex>
 
 namespace ripple {
 namespace test {
@@ -82,7 +82,13 @@ SuiteJournalSink::write(
 
     // Only write the string if the level at least equals the threshold.
     if (level >= threshold())
+    {
+        // std::endl flushes → sync() → str()/str("") race in shared buffer →
+        // crashes
+        static std::mutex log_mutex;
+        std::lock_guard lock(log_mutex);
         suite_.log << s << partition_ << text << std::endl;
+    }
 }
 
 class SuiteJournal