Make sure failed tests report their exit code (#240)

Commit 72c1660 introduced TestMain, but forgot to call os.Exit with the value returned from m.Run  This causes "go test" to exit with status 0, even if tests fail.  Adding os.Exit to TestMain makes sure the success/failure of tests is correctly reported through the exit code.

quick_test.go

@@ -34,7 +34,7 @@ func TestMain(m *testing.M) {
 	fmt.Fprintln(os.Stderr, "seed:", qseed)
 	fmt.Fprintf(os.Stderr, "quick settings: count=%v, items=%v, ksize=%v, vsize=%v\n", qcount, qmaxitems, qmaxksize, qmaxvsize)
 
-	m.Run()
+	os.Exit(m.Run())
 }
 
 func qconfig() *quick.Config {