misc: Increase issue 6997's test timeout to prevent spurious failures.

On heavily loaded build servers, a 5 second timeout is too aggressive,
which causes this test to fail spuriously.

LGTM=iant
R=iant
CC=golang-codereviews, sqweek
https://golang.org/cl/170850043

misc/cgo/test/issue6997_linux.go

@@ -34,7 +34,7 @@ func test6997(t *testing.T) {
 		if r == 0 {
 			t.Error("pthread finished but wasn't cancelled??")
 		}
-	case <-time.After(5 * time.Second):
+	case <-time.After(30 * time.Second):
 		t.Error("hung in pthread_cancel/pthread_join")
 	}
 }