dev: test schemas in ./src and ./schemas

[ralfhandl]

Part of

• Create dev build infrastructure #4223

Changes:

• separate test fixtures for published schemas in ./schemas and next schema in ./src
• one test script for all schema versions
• schema test coverage also for next schema in ./src

Tick one of the following options:

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request

[baywet]

from the meeting: we might want to name the directory 3.2 instead of vNext to match branching.
@handrews to have a look when brain is online :)

[handrews]

@ralfhandl I think I might just suggest calling "vNext" "dev" to match the branch. It's not clear to me that what's on the dev branch will always match the next version under development. In fact I feel like it's most likely to just be the same as whatever branch was most recently released as that's when cnanges are merged down to dev.

[ralfhandl]

I think I might just suggest calling "vNext" "dev"

@handrews "dev" for "under development", or "src" for "stuff in the src folder"?