client/windows integration tests: plumbing work for running on windows #1946

Job	Run time
prepare	1s
test / prepare	1m 58s
test / run (containerd, ./frontend/dockerfile, integration)	3m 32s
test / run (containerd, ./frontend/dockerfile, dockerfile)	3m 10s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	3m 26s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	3m 35s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	3m 4s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	3m 9s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 23s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 13s
test / run (oci, ./frontend/dockerfile, integration)	3m 18s
test / run (oci, ./frontend/dockerfile, dockerfile)	3m 46s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 2s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	3m 27s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 35s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 9s
image	0s
release	0s
	48m 48s

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