Control whether to run tests that depend on pandoc or LaTeX

Some of the export formats depend on pandoc or LaTeX, which are not trivial dependencies.

One way we can control whether to skip tests is to use custom command-line options to pytest: https://docs.pytest.org/en/latest/example/simple.html#pass-different-values-to-a-test-function-depending-on-command-line-options

This would allow someone without pandoc or LaTeX to run all of the other tests, and we could also skip tests on certain OS configurations in CI.