Refactor tests to use the new cmocka test runner

## Summary

Fixes https://github.com/profanity-im/profanity/issues/1907  

Update functional and unit test code to comply with the current cmocka test runner.  

## Changes

- `UnitTest` struct to `CMUnitTest` struct
- `unit_test()` macro to `cmocka_unit_test(f)` macro
- `unit_test_setup_teardown()` macro to `cmocka_unit_test_setup_teardown` macro
- `run_tests()` macro to `cmocka_run_group_tests()` function
- Setup and teardown functions return `int` instead of `void`

## Testing

### Unit Tests

`make check`

### Functional Tests

I did not compile or run functional tests because they are *shelved* for now.

### Valgrind

I'm not entirely sure how to fun Valgrind in this case. I did not do fancy memory management, so it should be fine.

tests/unittests/helpers.c

@@ -44,7 +44,7 @@ remove_data_dir(void** state)
     rmdir("./tests/files/xdg_data_home");
 }
 
-void
+int
 load_preferences(void** state)
 {
     create_config_dir(state);
@@ -53,29 +53,33 @@ load_preferences(void** state)
         prefs_load(NULL);
     }
     fclose(f);
+    return 0;
 }
 
-void
+int
 close_preferences(void** state)
 {
     prefs_close();
     remove("./tests/files/xdg_config_home/profanity/profrc");
     remove_config_dir(state);
     rmdir("./tests/files");
+    return 0;
 }
 
-void
+int
 init_chat_sessions(void** state)
 {
     load_preferences(NULL);
     chat_sessions_init();
+    return 0;
 }
 
-void
+int
 close_chat_sessions(void** state)
 {
     chat_sessions_clear();
     close_preferences(NULL);
+    return 0;
 }
 
 int