Add a unit test for `HTTPRequest`

[pafuent]

This PR aims to help "fix" https://github.com/godotengine/godot/issues/43440 It's adding cpp_mock header only mocking library to making unit test that requires mocks easier to implement.

I just implemented some really simple HTTPRequest unit tests that makes use of cpp_mock as an example of how to use it. My plan is to use this PR as a validation of cpp_mock and my approach to mock an HTTPClient instance that will be used by HTTPRequest on future unit tests.