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.

component/nxdtest/buildout.cfg

@@ -31,6 +31,6 @@ depends = ${python-prctl:egg}
 [nxdtest-repository]
 recipe  = slapos.recipe.build:gitclone
 repository = https://lab.nexedi.com/nexedi/nxdtest.git
-revision = 81b1907a
+revision = 6f75fa90
 location = ${buildout:parts-directory}/nxdtest
 git-executable = ${git:location}/bin/git