Updated tests in frontend/dockerfile/dockerfile_test.go to run on Win… #5289

Job	Run time
build (windows/amd64)	1m 43s
build (freebsd/amd64)	1m 47s
test-freebsd-amd64	2m 26s
test-windows-amd64 (containerd, ./client)	4m 6s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 9s
test-windows-amd64 (containerd, ./worker/containerd)	1m 56s
test-windows-amd64 (containerd, ./solver)	3m 8s
test-windows-amd64 (containerd, ./frontend)	3m 11s
test-windows-amd64 (containerd, ./frontend/dockerfile)	34m 47s
test-windows-amd64 (./..., 1)	7m 18s
	1h 3m 31s

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