ci: add imgtool test to workflows

Add imgtool test call to workflows, use pytest and
publish artifacts with test results.

Also enable test run on 'pull_request', but limit
the previous default imgtool_run.sh run ('environment' job)
to run on 'push' event only (the same behaviour as before).

Signed-off-by: Denis Mingulov <denis@mingulov.com>