Tests: allow reusing an existing module build
    
    Module builds take a long time to run, which can be a pain to wait for
    during integration test development.
    
    Instead of requiring developers to locally tweak the test code to be
    able to reuse module builds, allow specifying the build ID of the module
    build to be reused in test.env.yaml .
    
    Signed-off-by: Hunor Csomortáni <csomh@redhat.com>