integration tests and improved pytest suite

[kuefmz]

• pytests for different kinds of ontology failure classes (dns issue, http error, empty file, etc.)
• make above pytests reproducible by using a mock server that responds with those different failure categories #113
• Large-scale test for all archivo ontologies (requesting every ontology in different proxy modes and compare responses for verification)
• set proxy as system proxy (make sure that browsers also use system proxy) in https-only-for-archivo-ont-interception mode and timetravel to 10.10.2024 and then use it for over a week.
• use owlapi with proxy configured (probably via java) laoding a) an ontology that is not available and b) full imports where 1 imported onto is not available
• use it as a transparent proxy in a docker compose setup @JJ-Author
• select a list of terms from slash ontologies that are not available to check whether it works (probably best doing SPARQL) @JJ-Author

[kuefmz]

• also add testcase for HEAD request