v3.1-dev: schema tests use src/schemas/validation

[ralfhandl]

Run schema tests with current work-in-progress schema in src/schemas/validation folder

Open:

• Should the pass and fail folders with schema test instances also be copied into the src/schemas/validation folder?

[handrews]

@ralfhandl I'm now realizing that actually (per the update to CONTRIBUTING.md that I just made in #4186) that this (and, sadly, #4196 which I merged before thinking about it) should start on dev and then be merged out to the vX.Y-dev branches from there.

dev should be where everything fully works, so that each time we start a new vX.Y-dev branch from dev, it always gets the most up-to-date infrastructure. We never want to have to think about which branch is active or most current or whatever when starting new branches in the future. We just want to keep dev in good shape as the place where new branches always start.

[ralfhandl]

See

• dev: schema tests use src/schemas/validation #4198

[ralfhandl]

I cherry-picked the only commit in this PR for #4198.

Do we want to merge this PR, or open a new one updating v3.1-dev from dev?

[lornajane]

As discussed in TDC call, closing in favour of #4198