Merge branch 'feature/integrate_unit_tests' into 'master'

Merge branch 'feature/integrate_unit_tests' into 'master'

Integrate unit tests into build system

This MR moves unit tests from esp-idf-tests repository into 'test' subdirectories of respective components.

Tests are run using a runner app in tools/unit-test-app.

This needs a bit of cleanup:

1. remove extra newlines added to makefiles,
2. re-format unit tests code which has tabs
3. write a document on using this test app
4. maybe some refactoring in project.mk

I think 1&2&4 need to be done in this MR, while 3 may be done in a follow-up one.

See merge request !221