Control whether to run tests that depend on pandoc or LaTeX

[jayqi]

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.