test(bootstrapper): add unit tests for iterative phase handlers

Add direct unit tests for _phase_prepare_source, _phase_prepare_build,
_phase_build, and _phase_process_install_deps covering all branches
described in the upstream issue.

Closes: AIPCC-15986

Co-Authored-By: Claude <claude@anthropic.com>
Signed-off-by: Josef Skladanka <jskladan@redhat.com>

Author: 2 people