[v0.26] WCOW: Fix COPY --parents tests

[jsternberg]

As part of promoting COPY --parents to the stable syntax, I couldn't get the tests in dockerfile_parents_test.go to work on Windows. I think the feature should work on Windows, but I couldn't figure out how to get the test to run properly with Windows containers.

We need to fix that test to ensure it works properly on Windows and that COPY --parents works properly on Windows.

#6282

[profnandaa]

Adding here the previous file with the Windows tests for reference - https://github.com/moby/buildkit/blob/ec04db44738090800ccfab5d8b1147439bfd93e2/frontend/dockerfile/dockerfile_parents_test.go

[rzlink]

@jsternberg Currently, the test case TestCopyParents is already running in slice 10 of the dockerfile tests. However, the other two test cases — TestCopyRelativeParents and TestCopyParentsMissingDirectory — are currently skipped on Windows.

Does this task mean that we need to modify TestCopyRelativeParents and TestCopyParentsMissingDirectory in dockerfile_parents_test.go to ensure they work properly on the Windows platform?

[tonistiigi]

Does this task mean that we need to modify TestCopyRelativeParents and TestCopyParentsMissingDirectory in dockerfile_parents_test.go to ensure they work properly on the Windows platform?

Yes, these are the two skipped tests from #6282