Fix 'Error: Public Key required' when running flutterwave-node-v3 tests

## The problem

The flutterwave-node-v3 test suite fails at startup with `Error: Public Key required` from rave.base, before any test runs. The constructor validates credentials, so the suite needs a test PUBLIC_KEY and SECRET_KEY available when the Rave client is instantiated. Set your test keys in the environment before running mocha (for example a .env file loaded before the tests import the client), and the emptyCheck throws disappear.

## The fix

The test suite blew up with `Error: Public Key required` because no test credentials were configured. Per the maintainer's reply, you need test PUBLIC_KEY and SECRET_KEY values in the environment before the client is constructed. I set mine in a .env loaded at the top of the test setup and `npm test` started running. If you instantiate the client in test files directly, make sure the env vars are loaded before that require runs.