Move to testing mainly via BATS

[CasperWA]

Related to discussion in #3, particularly from this comment onwards.

Essentially we should explicitly test the action (mainly entrypoint.sh) does the expected thing, not that the server from optimade-python-tools can be validated.
This possibly involves changing the current CI tests to not setup a local server, but instead test public URLs, and expand the BATS tests to include all input parameters.

Input parameters to test in BATS:

• all versioned paths (implemented in Test unversioned base URLs #34)
• as type (implemented in Add verbosity and as_type input parameters #11)
• create output / results (implemented in Add create_outputs and results output #42)
• domain
• fail fast
• index
• path
• port
• protocol
• skip optional
• validator version (implemented in Add parameter to choose optimade-python-tools version, branch, tag or commit #4)
• verbosity (implemented in Add verbosity and as_type input parameters #11)