Allow marked tests to be skipped under Windows

Currently, if the test is marked with forked, this plugin tries to run it in a forked subprocess regardless of the current env. Also, adding a conditional skip marker does not help and manifests itself as https://github.com/pytest-dev/pytest/issues/7327.

The following snippet still runs on Windows:

@pytest.mark.skipif(IS_WINDOWS, ...)
@pytest.mark.forked
def test():
    ...

The workaround that I currently use is:

@pytest.mark.skipif(IS_WINDOWS, ...)
def test():
    ...

if not IS_WINDOWS:
    test = pytest.mark.forked(test)

This could be addressed in a few ways:

1. Actually respect skip markers.
2. Skip any tests marked with forked when the env is not supported.
3. Run these tests but w/o trying to sandbox them.

I think I like 1+2 most.

Start by reproducing the provided Windows example using the skipif and forked markers, and review the linked pytest issue 7327 for the marker interaction. Done should mean the selected behavior for unsupported Windows forked tests is implemented and verified against the reported snippet.