tests: Don't return an undefined value from main()

This caused test failures while backporting meson to an old runtime
environment.

Signed-off-by: Simon McVittie <smcv@collabora.com>

test cases/unit/17 prebuilt shared/patron.c

@@ -5,4 +5,5 @@ int main(int argc, char **argv) {
     printf("You are standing outside the Great Library of Alexandria.\n");
     printf("You decide to go inside.\n\n");
     alexandria_visit();
+    return 0;
 }