feat: add unit tests in ci

[maxday]

Description of changes:

• add latest version of gtest framework via CMakeLists.txt (1.15.2 as of today)
• add a noop.dummy_test, as a first step for having unit tests in CI
• update GitHub action workflow file to enable unit tests and run them
• update .clang-tidy not to complain on external testing dependencies

Testing:

• GitHub action CI is green when tests are passing
  
• GitHub action CI is red when tests are failing
  

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[marcomagdy]

@maxday why add a new unittest directory?
There is already a test directory with a CMake file and all.

[maxday]

@maxday why add a new unittest directory? There is already a test directory with a CMake file and all.

It seems to me that existing tests are integration tests (which requires a more complex infra to be able to run them with CodeBuild and the SDK).
This new repo will contain unit tests, with an extremely fast feedback loop. This will help testing the runtime at the function level, without any third party integration.
Does it make sense to you?

[marcomagdy]

Can you think of a way to combine both tests?
That would help us use the same google-test library for both unit and integration tests. We can even use the same CMake file potentially with some flags to turn on/off the integration tests.

It's not ideal to we have two different sets of tests that use different versions of the testing library.