Improving testmon cache

[ver217]

Overview

CI uses single shared testmon cache now, and it stores testmon cache when unit tests finish, so PRs will affect each other.

E.g. PR 1 changes src/a.py and it's testmon cache is stored. PR 2 is created before PR 1 is merged, and it changes src/b.py. PR 2 will restore the latest testmon cache, which caches src/a.py changed in PR 1, thus PR 2 will run tests related to src/a.py and src/b.py. In fact, PR 2 does not need to run tests related to src/a.py. This is how PRs affect each other.

Possible solution

We can manage cache for every branch and every PR.

1. When creating a new branch, we copy cache/main/.testmondata* to cache/<branch>/.
2. When creating a new PR or change the base branch, we copy cache/<base_ref>/.testmondata* to cache/_pull/<pr_number>/.
3. When running unit tests for each PR, we restore testmon cache from cache/_pull/<pr_number>/.
4. When a PR is closed, if it's merged, we copy cache/_pull/<pr_number>/.testmondata* to cache/<base_ref>/. Otherwise, we just remove cache/_pull/<pr_number>.
5. When a branch is deleted, we remove cache/<ref>.

In this way, unmerged PRs won't affect each other anymore. And to avoid store outdated testmon cache, we force each branch is up-to-date before merging.