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/test_muc.c

@@ -6,16 +6,18 @@
 
 #include "xmpp/muc.h"
 
-void
+int
 muc_before_test(void** state)
 {
     muc_init();
+    return 0;
 }
 
-void
+int
 muc_after_test(void** state)
 {
     muc_close();
+    return 0;
 }
 
 void