tests: with_mcumgr: Rework to use prebuilt signed images

The pytest scenarios relied on the west sign command to
re-sign images at runtime, which broke after that command
changed. Rework the tests to use Twister's
required_applications feature instead.

All upgrade images are now described in testcase.yaml,
built by Twister with the desired versions and signature
keys, and passed to pytest as prebuilt artifacts. The
pytest code no longer signs or modifies images; it only
uploads the prebuilt binaries and verifies the boot
behaviour.

Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>