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.h

@@ -1,10 +1,10 @@
 #include "glib.h"
 
-void load_preferences(void** state);
-void close_preferences(void** state);
+int load_preferences(void** state);
+int close_preferences(void** state);
 
-void init_chat_sessions(void** state);
-void close_chat_sessions(void** state);
+int init_chat_sessions(void** state);
+int close_chat_sessions(void** state);
 
 int utf8_pos_to_col(char* str, int utf8_pos);