Adding support of googletest / googlemock

[anobli]

Introduction

Zephyr already provide an awesome test suite that work very well with C but seems more limited with C++.
It would be great having googletest and googlemock available in Zephyr for testing complex C++ libraries or applications.

Problem description

I guess the major issue is mocking a C++ class is not easily doable and is quite limited with the current test suite.
In addition, gmock and gtest are widely used, supporting it in zephyr would simplify writing C++ tests.

Proposed change

Create a googletest external module that could be used to write tests using gmock and gtest.

Proposed change (Detailed)

I would to update googletest to support Zephyr, update it to make it an external module.
I guess that Zephyr OS support itself should accepted upstream, not sure about the external module changes so we may have to fork googletest.
I would also adds some unit tests example written with gtest to demonstrate how using it with twister, and also, to make sure the test suite is working correctly.
There is also some changes to do in gtest harness (I need to check if this still the case on master) . I am not sure why, it was not able to parse the tests output.

Dependencies

This would add a new component, googletest to zephyr. I think we could make it optional since I think it should only downloaded if this is actually used by a project. Ideally, all the required changes in googletest would be upstream. If this is not the case, we may to fork it.

Alternatives

I have looked for some alternatives for testing / mocking C++ code but I have not found anything already supported by Zephyr.
In addition, I am working on projects already using it.

[github-actions]

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[JeffWelder]

I flipped you a DM on discord about my attempts to support showing these tests in the VSCode C++ Testmate extension. 😀

[kartben]

@anobli Twister already has a gTest harness, have you tried it?

[anobli]

@kartben Yes, I have been using the gTest harness, and it was working. The only issue I had was with the timings printed by gTest (the harness doesn't handle them).
I should have precised it, but, the the issue not really with gTest but really with gmok.
IIRC, gmock only build for native target, but, it doesn't build for any qemu or real target.
I have opened a PR for it, but, it take a while:
google/googletest#4692

[kartben]

@yperess might be able to help