Cleanup empty nontmp_workdir.

A failed test leaves currently two directories, one in tmp and one in
the build directory. The latter seems to stay for most tests empty.
Therefore attempt to remove this directory if it is empty.

src/test/util.sh

@@ -87,7 +87,8 @@ function onexit {
             rm -rf $nontmp_workdir
         fi
     else
-        echo Test $TESTNAME failed, leaving behind $tmp_workdir and $nontmp_workdir
+        echo -n Test $TESTNAME failed, leaving behind $tmp_workdir
+        (rmdir "$nontmp_workdir" 2>/dev/null && echo) || echo " and $nontmp_workdir"
         echo To replay the failed test, run
         echo " " _RR_TRACE_DIR="$workdir" rr replay
         exit 1