Reduce the tightness of AddMSecsWorks() bounds.

The time elapsed is in the range 30-34 on a Gentoo system.

As long as the elapsed time is between 25 and 51 ms the addMSecs call did work,
it may just take longer to get our timeslice back on some systems than others.

mythtv/libs/libmythbase/test/test_mythtimer/test_mythtimer.h

@@ -98,7 +98,7 @@ class TestMythTimer: public QObject
         t.start();
         t.addMSecs(-25);
         usleep(52 * 1000);
-        QVERIFY(t.elapsed() > 25 && t.elapsed() < 30);
+        QVERIFY(t.elapsed() > 25 && t.elapsed() < 50);
     }
 
     void AddMSecsIsResetOnStart(void)