Replace unmaintained ajv-cli with custom script

[senekor]

closes #2283

ajv-cli is unmaintained. It gets flagged with a security vulnerability and a PR which would fix that has existed without response for close to a year.

The underlying library ajv seems to be less unmaintained. It had its last release about nine months ago.

[senekor]

Note on performance: yarn test-one and yarn test take almost the same amount of time (0.35 / 0.38 seconds) so I'm guessing startup of node / yarn dominates. I'd say the simple synchronous loop in the script should be fine.

[senekor]

CI fix in separate PR: #2353