Remove pandoc formats from integration tests

[jochenklar]

I think it would suffice to test the different pandoc formats once in a unit test (rdmo/core/tests/test_pandoc.py) and remove the parametrization from the other tests (and use only html as format). This should save some time.

[afuetterer]

I think the most expensive scenario is the pdf generation.

[afuetterer]

Related #1092

[jochenklar]

Great, mocking the test is actually more elegant than just leaving it out.

[afuetterer]

See related PR #1117, let's discuss details in the PR, alright?

[jochenklar]

@MyPyDavid we can close this, right?

[MyPyDavid]

I think so, because it was merged with #1319 right?