fixup! component/nxdtest: v↑ (namespaces, /tmp-on-tmpfs, cancellation)

Preserve "disk" group to stay in /etc/group as explained in
nxdtest@6f75fa90 because
slapos.core tests use this.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

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failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

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