test: exit non-zero on error from nilptr2.go.

(For go.tools/ssa/interp/interp_test, which runs a subset of
these tests under this assumption.)

R=rsc, r
CC=golang-dev
https://golang.org/cl/13108043

test/nilptr2.go

@@ -6,6 +6,8 @@
 
 package main
 
+import "os"
+
 func main() {
 	ok := true
 	for _, tt := range tests {
@@ -21,6 +23,7 @@ func main() {
 	}
 	if !ok {
 		println("BUG")
+		os.Exit(1)
 	}
 }