Make test_conf return failure status on failure!

Ahem. Of course I've been running it interactively until now, so I
never noticed that I'd forgotten to fill in that important point. But
now it's run as part of my build, it should make sure to fail if it
fails.

test/test_conf.c

@@ -769,5 +769,5 @@ void test_simple(void)
 int main(void)
 {
     test_simple();
-    return 0;
+    return nfails != 0;
 }