MACAddress: Unit testing for basic functionality

Problem:
- There are no unit tests for `MACAddress` class. This makes it
  difficult to refactor and ensure the same behavior.
- `m_data` private member variable is uninitialized leading to undefined
  behavior of `is_zero()`.

Solution:
- Add unit tests to cover basic functionality.
- Initialize `m_data`.

AK/MACAddress.h

@@ -71,7 +71,7 @@ public:
     }
 
 private:
-    u8 m_data[6];
+    u8 m_data[6] {};
 };
 
 static_assert(sizeof(MACAddress) == 6);