Proposal: block merging when pull request has none test tasks checked

What

Let ti-chi-bot block merging when pull request has none test tasks checked.

Why

Current there’s some required checks in Github pull request template of pingcap/tidb repo.

### Check List

Tests <!-- At least one of them must be included. -->

- [ ] Unit test
- [ ] Integration test
- [ ] Manual test (add detailed scripts or steps below)
- [ ] No code

Side effects

- [ ] Performance regression: Consumes more CPU
- [ ] Performance regression: Consumes more Memory
- [ ] Breaking backward compatibility

Documentation

- [ ] Affects user behaviors
- [ ] Contains syntax changes
- [ ] Contains variable changes
- [ ] Contains experimental features
- [ ] Changes MySQL compatibility

But there’s a large proportion of pull requests that has none options checked in Tests section.

How

We can use TiChi’s format checker plugin to reach it.
I contributed a pull request to ti-chi-bot’s configuration repo: https://github.com/ti-community-infra/configs/pull/953

PTAL, Comments and suggestions welcome. :smiley: