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

This upgrade

• teaches nxdtest to run each testcase with its own /tmp and /dev/shm to detect after each test run leaked temporary files, leaked mount entries, to isolate different test runs from each other, and to provide tmpfs on /tmp for every test.

  https://lab.nexedi.com/nexedi/nxdtest/merge_requests/13

• teaches nxdtest to propagate cancellation to spawned test jobs, so that whenever user cancels corresponding test run in ERP5 UI, nxdtest stops promptly. This should be most useful for long-running tests like SlapOS.SoftwareReleases.IntegrationTest-* which before had to be waited for for hours to complete even after cancelling test run in ERP5.

  https://lab.nexedi.com/nexedi/nxdtest/merge_requests/14

The first patch consolidates pinning of mock egg.
The second patch does nxdtest upgrade itself.

/cc @tomo, @jerome