BUG: fix testing *rst tutorials

[ev-br]

• get optionflags from DTConfig
• allow pytest_extra_skips in collection

This is a minimal fix to close #130: the following passes on scipy

$ python dev.py shell
$ cd scipy/doc/source/tutorials
$ pytest doc/source/tutorial/ --doctest-glob=*rst

Further enhancements are tracked in #132, #56.

Further UX improvements:

• why is --doctest-glob=*rst needed? Looks like there's an issue in https://github.com/pytest-dev/pytest/blob/8.0.2/src/_pytest/doctest.py#L138 and https://github.com/pytest-dev/pytest/blob/8.0.2/src/_pytest/doctest.py#L151 --- figure out or report upstream?
• cook up a nicer UX for dev.py: with refguide-check it was "skip tutorials if -s submodule, run them otherwise". Not nice.

[ev-br]

Am going to land this as a minimal viable implementation. A refactor needed for gh-132 is next.