update CI config: caching of repo, venv, split test install/run

[protolambda]

• repo itself is cached now, for faster checkout
• venv for test running is cached now
• venv is installed first now, and tests run separately. Also makes local runs faster if you don't need to re-install dependencies (even if they are cached)
• make citest does the same as make test, but outputs a test-report for CI
  • Note: the test reporting can be improved, we would want one separate report for each sub-topic. But the tests need to be more structured for that first. This is at least an improvement on status-quo.

[protolambda]

@djrtwo LGTM?