Split the pull request tests into the same six shards as the release tests #4144

Problem

The pull request workflow's Test job runs the whole make test suite on one runner. On #4140's last run it took 28 minutes (13:58 to 14:26 UTC). The release run already splits the same suite over six runners (#4134), where each shard finishes in 3 to 5 minutes.

The split also runs only on release commits, so a pull request can merge with a change that breaks it. That is what happened in release run 41063091599: a test that starts pytest itself failed only on shard 6 (#4139), and nothing ran the split before the merge.

Proposal

• pr.yml's test job becomes a matrix of six shards (fail-fast: false). Each shard runs make test with the shard plugin loaded through PYTEST_ADDOPTS, as .github/scripts/run_release_tests.sh does for push.yml's release-tests job, and uploads the tests it collected and ran.
• A new job downloads every shard's report and runs python -m scripts.release_test_shards check-coverage, which fails unless each collected test ran in exactly one shard. CI complete needs it.
• The shards' coverage reports go to Codecov in one upload from a job that needs every shard, so Codecov gets one upload for the test suite, as before the split.
• A test checks that pr.yml and push.yml use the same shards.

The cross-database integration tests took 29 minutes on the same run, so the pull request workflow stays about that long until that job is also faster.