Initialize ReusableStringStream cache before user threads can run

The initialization itself is thread unsafe, and as such we cannot
allow it to be delayed until multiple user-spawned threads need it.

src/catch2/internal/catch_run_context.cpp

@@ -210,6 +210,13 @@ namespace Catch {
     {
         getCurrentMutableContext().setResultCapture( this );
         m_reporter->testRunStarting(m_runInfo);
+
+        // TODO: HACK!
+        //       We need to make sure the underlying cache is initialized
+        //       while we are guaranteed to be running in a single thread,
+        //       because the initialization is not thread-safe.
+        ReusableStringStream rss;
+        (void)rss;
     }
 
     RunContext::~RunContext() {