test: ensure all failing tests exit nonzero.

Previously merely printing an error would cause the golden
file comparison (in 'bash run') to fail, but that is no longer
the case with the new run.go driver.

R=iant
CC=golang-dev
https://golang.org/cl/7310087
diff --git a/test/func8.go b/test/func8.go
index 7defe26..1305180 100644
--- a/test/func8.go
+++ b/test/func8.go
@@ -37,13 +37,13 @@
 
 func main() {
 	if f() == g() {
-		println("wrong f,g order")
+		panic("wrong f,g order")
 	}
 
 	if x() == (y() == "abc") {
 		panic("wrong compare")
 	}
 	if xy != "xy" {
-		println("wrong x,y order")
+		panic("wrong x,y order")
 	}
 }