Automatically generate test meta

[MonsieurNicolas]

This PR adds a new Github action (to be installed in the contributor's fork) that automatically adds the updates to the "test meta" in the background.

This makes it much easier and faster as contributors do not need to

• remember to generate the updates
• build both current and vnext and generate each

(this replaces #3401 )

[MonsieurNicolas]

Hold onto merging this: there is some unexpected meta update that I am still chasing

[MonsieurNicolas]

This is now good to go: the issue was that the test infra to run tests was only running a subset of tests per file (because of batching), which caused the meta to be only contain data from the last batch for each test file.

[sisuresh]

r+ b74fe83

[MonsieurNicolas]

this is blocked on a change in bors (should allow "skipped" workflow, and only look at failed ones)