Jenkinsfile: also run unit-tests on rootless

[thaJeztah]

Not all of our unit tests are fully "unit", and some may depend on the
environment in which they're running, so let's also run these in the
rootless stage of our Jenkinsfile.

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Looks like the fuse-overlayfs tests are skipped on rootless (probably something we should fix, if possible);

=== SKIP: daemon/graphdriver/fuse-overlayfs TestFUSEOverlayFSSetup (0.00s)
time="2021-08-12T12:48:15Z" level=error msg="exec: \"fuse-overlayfs\": executable file not found in $PATH" storage-driver=fuse-overlayfs
    graphtest_unix.go:42: graphdriver: driver not supported
    graphtest_unix.go:44: Driver fuse-overlayfs not supported

=== SKIP: daemon/graphdriver/fuse-overlayfs TestFUSEOverlayFSCreateEmpty (0.00s)
time="2021-08-12T12:48:15Z" level=error msg="exec: \"fuse-overlayfs\": executable file not found in $PATH" storage-driver=fuse-overlayfs
    graphtest_unix.go:42: graphdriver: driver not supported
    graphtest_unix.go:44: Driver fuse-overlayfs not supported

=== SKIP: daemon/graphdriver/fuse-overlayfs TestFUSEOverlayFSCreateBase (0.00s)
time="2021-08-12T12:48:15Z" level=error msg="exec: \"fuse-overlayfs\": executable file not found in $PATH" storage-driver=fuse-overlayfs
    graphtest_unix.go:42: graphdriver: driver not supported
    graphtest_unix.go:44: Driver fuse-overlayfs not supported

=== SKIP: daemon/graphdriver/fuse-overlayfs TestFUSEOverlayFSCreateSnap (0.00s)
time="2021-08-12T12:48:15Z" level=error msg="exec: \"fuse-overlayfs\": executable file not found in $PATH" storage-driver=fuse-overlayfs
    graphtest_unix.go:42: graphdriver: driver not supported
    graphtest_unix.go:44: Driver fuse-overlayfs not supported

=== SKIP: daemon/graphdriver/fuse-overlayfs TestFUSEOverlayFS128LayerRead (0.00s)
time="2021-08-12T12:48:15Z" level=error msg="exec: \"fuse-overlayfs\": executable file not found in $PATH" storage-driver=fuse-overlayfs
    graphtest_unix.go:42: graphdriver: driver not supported
    graphtest_unix.go:44: Driver fuse-overlayfs not supported

=== SKIP: daemon/graphdriver/fuse-overlayfs TestFUSEOverlayFSTeardown (0.00s)
    graphtest_unix.go:71: No driver to put!

Also skipping some overlay/overlay2 tests (do we need those?)

=== SKIP: daemon/graphdriver/overlay TestOverlayDiffApply10Files (0.00s)
    overlay_test.go:44: Fails to compute changes after apply intermittently

=== SKIP: daemon/graphdriver/overlay TestOverlayChanges (0.00s)
    overlay_test.go:49: Fails to compute changes intermittently

=== SKIP: daemon/graphdriver/overlay2 TestOverlayChanges (0.00s)
    overlay_test.go:65: Cannot run test with naive change algorithm

We need to check if these are running elsewhere;

=== SKIP: cmd/docker-proxy TestTCP6Proxy (0.00s)
    network_proxy_test.go:197: Need to start CI docker with --ipv6

=== SKIP: cmd/docker-proxy TestTCPDualStackProxy (0.00s)
    network_proxy_test.go:212: No support for dual stack yet

=== SKIP: cmd/docker-proxy TestUDP6Proxy (0.00s)
    network_proxy_test.go:241: Need to start CI docker with --ipv6

=== SKIP: cmd/docker-proxy TestSCTP6Proxy (0.00s)
    network_proxy_test.go:302: Need to start CI docker with --ipv6
``

[AkihiroSuda]

I don’t think this env is consumed by unit tests

[AkihiroSuda]

I guess we can execute rootlesskit go test in hack/make/unit when this env var is set.

rootlesskit flags could be copied from dockerd-rootless.sh