Add supportfor multiple skip/xfail files

[betatim]

This allows you to specify several files which are combined. For examplea general file plus a platform specific one.

You can now do something like pytest --skips-file skips-general.txt --skips-file skips-macos.txt array_api_tests/.

There was a difference in line handling between skips and xfails. I unified them to how xfails handles things. The skips lines were only taken if line.startswith("array_api_tests"). And all other lines were ignored. Now all lines that aren't blank or start with a # are kept. They still have to resolve to valid test IDs, so I think the end result will be the same.

This came out of a discussion at data-apis/array-api-strict#240 (comment)

[betatim]

I'm not sure who should/can review this, so I requested a review from @ev-br because GitHub suggested it.

How do people generally decide on merging (or not) things in this repo? Wait for one review? One thumbs up from Lucas and then self-merge?

[ev-br]

Test-driving this PR locally:

1. The following incantation fails on master and passes in this PR. This is the expected correct behavior: the test is xfailed in the numpy-xfails.txt, so in master two xfail files are not understood, and in this PR they are.

$ ARRAY_API_TESTS_MODULE=numpy pytest array_api_tests/test_data_type_functions.py::test_finfo[float32]  --skips-file=../array-api-compat/numpy-xfails.txt --skips-file=../array-api-strict/array-api-tests-xfails.txt  -v

2. Running with no skips skill works: the following incantation fails as expected

$ ARRAY_API_TESTS_MODULE=numpy pytest array_api_tests/test_data_type_functions.py::test_finfo[float32] 

The code change LGTM, thank you @betatim

[ev-br]

How do people generally decide on merging (or not) things in this repo? Wait for one review? One thumbs up from Lucas and then self-merge?

For the last year or so, it was mostly me self-merging at ad hoc times.
Not that it's healthy or desired, and I'd be very happy for more hands and eyes :-).