tests: Replace returns with exits in the main functions

This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

tests/stdlib/a64l.c

@@ -6,7 +6,7 @@ int main(void) {
     long l = a64l(s);
     if (l != 194301926) {
         printf("Invalid result: a64l(%s) = %ld\n", s, l);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     printf("Correct a64l: %s = %ld\n", s, l);
 
@@ -15,7 +15,7 @@ int main(void) {
     l = a64l(s);
     if (l != 53222) {
         printf("Invalid result: a64l(%s) = %ld\n", s, l);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     printf("Correct a64l: %s = %ld\n", s, l);
 }

tests/stdlib/bsearch.c

@@ -13,7 +13,7 @@ int int_cmp(const void* a, const void* b) {
       size_t i = 0; \
       for (; i < len; ++i) printf("%d,", arr[i]); \
       printf("] expected %p but got %p\n", (void*) expect, res); \
-      return EXIT_FAILURE; \
+      exit(EXIT_FAILURE); \
     } \
   } while (0);
 

tests/stdlib/env.c

@@ -34,7 +34,7 @@ int main(void) {
     if (env) {
         puts("This should be null, but isn't");
         puts(env);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     } else {
         puts("Value deleted successfully!");
     }

tests/stdlib/realpath.c

@@ -8,7 +8,7 @@ int main(void) {
     char* path = realpath("stdlib/realpath.c", NULL);
     if (!path) {
         perror("realpath");
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     puts(path);
 
@@ -21,7 +21,7 @@ int main(void) {
     if (!path) {
         perror("realpath");
         free(path);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     puts(path);