tests: rename default plan to build-gating to better eflect its current purpose
    
    My expectation is that in the future, more tests will land in this git,
    and not all of them would be useful in gating time. AFAIK, CI systems do
    run everything they can, therefore adding `when` to control when the
    plan is executed - `is not defined` bit should allow anyone to run it
    manually, while the rest of the condition should limit its presence to
    gating events.