client: improve releasable condition in tests #5414

Job	Run time
build (windows/amd64)	1m 42s
build (freebsd/amd64)	1m 52s
test-freebsd-amd64	2m 36s
test-windows-amd64 (containerd, ./client)	3m 59s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 22s
test-windows-amd64 (containerd, ./worker/containerd)	1m 43s
test-windows-amd64 (containerd, ./solver)	1m 55s
test-windows-amd64 (containerd, ./frontend)	2m 24s
test-windows-amd64 (containerd, ./frontend/dockerfile)	32m 34s
test-windows-amd64 (./..., 1)	4m 14s
	55m 21s

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