Update UTs needing test_executor_single_thread to use it explicitly.

Prep for changing G42CORE_TEST_RUN_TESTS to use tests_executor.
Unit test test_executor_multiple_threads with test part for primary thread.
G42CORE_TEST_RUN_TESTS updated to use test_execution.
A few fixes to test_execution and a unit test for issues discovered via integration.

Move trivialtests_detail_public_ut_required_api*.cpp up on directory out of bootstrap060.

Add bootstrap070 and include in master solution.

Update bootstrap README.