tracing/tracetest: Mark T failed on fatalfInNonMain

There is no guaranty that the main thread would call Fatal or Fail if a
non-main thread calls fatalfInNonMain. This way it was possible for a
test to succeed if only non-main thread(s) failed somehow.

-> Fix it by marking the test as failed if any non-main thread fails.

tracing/tracetest/tracetest.go

@@ -512,5 +512,6 @@ func (t *T) fatalfInNonMain(format string, argv ...interface{}) {
 
 	t.Logf(format, argv...)
 	t.Logf("%s\n", debug.Stack())
+	t.Fail()
 	runtime.Goexit()
 }