Cumulus test-parachain node and test runtime were still using relay
chain consensus and 12s blocktimes. With async backing around the corner
on the major chains we should switch our tests too.

Also needed to nicely test the changes coming to collators in #3168.

### Changes Overview
- Followed the [migration
guide](https://wiki.polkadot.network/docs/maintain-guides-async-backing)
for async backing for the cumulus-test-runtime
- Adjusted the cumulus-test-service to use the correct import-queue,
lookahead collator etc.
- The block validation function now uses the Aura Ext Executor so that
the seal of the block is validated
- Previous point requires that we seal block before calling into
`validate_block`, I introduced a helper function for that
- Test client adjusted to provide a slot to the relay chain proof and
the aura pre-digest