runtests: Disable valgrind when debugging

This was already mostly being done, except that analysis after the
test still assumed that the valgrind log files would be available. An
alternative way to handle the valgrind + gdb combination could be to
enable one of the valgrind debugger hooks.

tests/runtests.pl

@@ -3334,6 +3334,13 @@ sub singletest {
         $DBGCURL=$CMDLINE;
     }
 
+    if($gdbthis) {
+        # gdb is incompatible with valgrind, so disable it when debugging
+        # Perhaps a better approach would be to run it under valgrind anyway
+        # with --db-attach=yes or --vgdb=yes.
+        $disablevalgrind=1;
+    }
+
     if($fail_due_event_based) {
         logmsg "This test cannot run event based\n";
         return -1;