Make tidb test suite consume less disk space, support automated runs

## Enhancement
The test suite consumes an enormous about of disk space: when I run it on a VM limited to 14GB, it exhausts disk space and then dies. A great deal of space is taken up by the build cache alone. As I watch the executor tests run right now, the build cache is using just under 4GB of space.

I propose to make the test suite use less disk space, so it can run on space-constrained hosts. In particular, I have a working Github automation that automatically runs when, e.g., a PR is merged, but it fails every time because it's limited to 14GB of disk space.

The change I propose will:

- Conserve disk space by running "go clean -cache" between groups of tests,
- Keep going when one group of tests fails (but still ultimately fail in that case)
- Provide a Github action to automatically run tests