[Test] Implementing unit tests for `SyncedEnforcer`

### Description

[Functional tests](https://github.com/casbin/casbin-cpp/blob/master/tests/enforcer_synced_test.cpp) for `casbin::SyncedEnforcer`. The problem arose due to ambiguity in handling multi-threaded tests in GoogleTest.

We need to inspect ways to get around this and implement it in our current test setup.

### Your System Info

NA

### Screenshots/Testimonials

NA
