[subsystem-bench] Add mocks for own assignments triggering (#5042)

Since https://github.com/paritytech/polkadot-sdk/pull/4772, the
benchamrks triggers its own assignments, but the mocks weren't properly
hooked up, so that the approval is sent as well, so they would have
counted as no-shows and impact the time it takes for a block to be
approved.

Fixed by adding mocks for availability recovery and candidate-validation
and hook those into the benchmark.

Signed-off-by: Alexandru Gheorghe <[email protected]>