imgtool: initial sanity test (for CI build)

[mingulov]

imgtool tests are not used by CI (workflows), so even existing tests might be broken (or some other functionality, as described in #1769).

Modified workflows to use existing imgtool unittests also trigger it on 'pull_request'. An initial sanity test for imgtool commands is added. As some already existing unittests are broken, fix them to be able to start to use this change. Added missing dependencies to setup.py / requirements.txt (it also prevented sanity test execution).

An example for CI build can be found here.