libct/int: skip TestUsernsCheckpoint on CentOS 7 #6219

Job	Run time
cross-i386	1m 26s
test (ubuntu-20.04, 1.20.x, rootless, -race)	6m 35s
cross-i386 (runc_nodmz)	1m 23s
test (ubuntu-20.04, 1.20.x, rootless)	6m 6s
test (ubuntu-20.04, 1.20.x, -race)	7m 0s
test (ubuntu-20.04, 1.20.x)	6m 28s
test (ubuntu-20.04, 1.21.x, rootless, -race)	6m 24s
test (ubuntu-20.04, 1.21.x, rootless)	6m 4s
test (ubuntu-20.04, 1.21.x, -race)	7m 3s
test (ubuntu-20.04, 1.21.x)	6m 35s
test (ubuntu-20.04, 1.21.x, criu-dev)	7m 2s
test (ubuntu-24.04, 1.20.x, rootless, -race)	10m 26s
test (ubuntu-24.04, 1.20.x, rootless)	9m 13s
test (ubuntu-24.04, 1.20.x, -race)	8m 11s
test (ubuntu-24.04, 1.20.x)	7m 24s
test (ubuntu-24.04, 1.21.x, rootless, -race)	9m 25s
test (ubuntu-24.04, 1.21.x, rootless)	9m 6s
test (ubuntu-24.04, 1.21.x, -race)	8m 6s
test (ubuntu-24.04, 1.21.x)	6m 45s
test (ubuntu-24.04, 1.21.x, runc_nodmz)	6m 54s
test (ubuntu-24.04, 1.21.x, criu-dev)	7m 23s
test (actuated-arm64-6cpu-8gb, 1.21.x, rootless)	15m 0s
test (actuated-arm64-6cpu-8gb, 1.21.x)	8m 31s
	2h 48m 30s

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