testsuite: coveerage: support counters reset

Ability to reset gcov counters allows better isolation of tests
coverage. Specifically it allows to:
1. Not include counters for any code that was executed prior running the
   test, which includes all the code executed during the board boot.
2. Run multiple tests without firmware reload by resetting counters
   between each.

Signed-off-by: Roman Studenikin <srv@meta.com>