Make the concurrency-group assert independent of the template

Checking only for `concurrency:` and `github.event.pull_request.number` still
passed after `cancel-in-progress: true` was dropped, and after the head sha was
dropped from the group, because the sha also reaches the rendered file through
`CHECKOUT_REF`. Comparing against `TEMPLATE_CONCURRENCY_PR` itself is no
better: mutating the template mutates both sides of that comparison, so
deleting `cancel-in-progress: true` from it leaves `praktika yaml` at exit 0
and a rendered `pull_request.yml` with no such line.

Spelling the expected block out at the assert makes each of the four mutations
that break the contract fail generation: `{CONCURRENCY}` removed from
`TEMPLATE_PULL_REQUEST_0`, `cancel-in-progress` removed, the head sha removed,
the pull request number removed.