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/const4.go b/test/const4.go
index 677fcef..2fb2d06 100644
--- a/test/const4.go
+++ b/test/const4.go
@@ -9,7 +9,7 @@
 package main
 
 var b struct {
-	a[10]int
+	a [10]int
 }
 
 var m map[string][20]int
@@ -61,17 +61,22 @@
 func main() {
 	if n1 != 10 || n2 != 20 || n3 != 30 || n4 != 40 || n5 != 50 || n6 != 60 || n7 != 70 {
 		println("BUG:", n1, n2, n3, n4, n5, n6, n7)
+		panic("fail")
 	}
 	if !calledF {
 		println("BUG: did not call f")
+		panic("fail")
 	}
 	if <-c == nil {
 		println("BUG: did not receive from c")
+		panic("fail")
 	}
 	if !calledG {
 		println("BUG: did not call g")
+		panic("fail")
 	}
 	if <-c1 == nil {
 		println("BUG: did not receive from c1")
+		panic("fail")
 	}
 }