tests: make main function default to fail status

Before this commit, calling the test executable with wrong arguments
causes it to only print an error/usage message, but actual exit code was
0 (success). This is not the intended behaviour. This commit fixes it.

Author: giomba

src/main_test.c

@@ -3,7 +3,7 @@
 #include <stdio.h>
 
 int main(int argc, char *argv[]) {
-    int ret = 0;
+    int ret = 1;
 
     printf("CEDA Test\n");
     criterion_options.color = true;