test cases: Don't fall off the end of main() without an exit status

This is undefined behaviour, and seems to have caused test failures
when backporting Meson to an older toolchain in the Steam Runtime.

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

test cases/common/141 special characters/arg-char-test.c

@@ -7,4 +7,5 @@ int main(int argc, char **argv) {
   if (c != argv[1][0])
     fprintf(stderr, "Expected %x, got %x\n", (unsigned int) c, (unsigned int) argv[1][0]);
   assert(c == argv[1][0]);
+  return 0;
 }

test cases/common/141 special characters/arg-string-test.c

@@ -9,4 +9,5 @@ int main(int argc, char **argv) {
   if (s[0] != argv[1][0])
     fprintf(stderr, "Expected %x, got %x\n", (unsigned int) s[0], (unsigned int) argv[1][0]);
   assert(s[0] == argv[1][0]);
+  return 0;
 }

test cases/common/141 special characters/arg-unquoted-test.c

@@ -14,4 +14,5 @@ int main(int argc, char **argv) {
   assert(s[0] == argv[1][0]);
   // There is no way to convert a macro argument into a character constant.
   // Otherwise we'd test that as well
+  return 0;
 }