initialise capabilities unit test

[aojea]

/kind failing-test
/kind flake

NONE

The test needs to initialise the variable, or subsequent testrun carry over the previous state.
Before:

=== FAIL: pkg/capabilities TestGet (0.00s)
    capabilities_test.go:36: expected Capabilities: capabilities.Capabilities{AllowPrivileged:false, PrivilegedSources:capabilities.PrivilegedSources{HostNetworkSources:[]string{}, HostPIDSources:[]string{}, HostIPCSources:[]string{}}, PerConnectionBandwidthLimitBytesPerSec:0}, got a non-default: capabilities.Capabilities{AllowPrivileged:false, PrivilegedSources:capabilities.PrivilegedSources{HostNetworkSources:[]string{"A", "B"}, HostPIDSources:[]string(nil), HostIPCSources:[]string(nil)}, PerConnectionBandwidthLimitBytesPerSec:0}
    capabilities_test.go:36: expected Capabilities: capabilities.Capabilities{AllowPrivileged:false, PrivilegedSources:capabilities.PrivilegedSources{HostNetworkSources:[]string{}, HostPIDSources:[]string{}, HostIPCSources:[]string{}}, PerConnectionBandwidthLimitBytesPerSec:0}, got a non-default: capabilities.Capabilities{AllowPrivileged:false, PrivilegedSources:capabilities.PrivilegedSources{HostNetworkSources:[]string{"A", "B"}, HostPIDSources:[]string(nil), HostIPCSources:[]string(nil)}, PerConnectionBandwidthLimitBytesPerSec:0}

With current change:

[aojea@aojea-laptop kubernetes]$ go test -timeout 300s -count 300  ./pkg/capabilities/ -run .
ok      k8s.io/kubernetes/pkg/capabilities      0.004s

xref: https://testgrid.k8s.io/sig-testing-canaries#ci-kubernetes-generate-make-test-count10

[k8s-ci-robot]

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[aojea]

/assign @liggitt @spiffxp

[aojea]

/sig testing

[liggitt]

/lgtm
/approve

/milestone v1.21

test-only change to fix a failing test in the flake flushing job

[k8s-ci-robot]

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