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.

  nxdtest!13 (merged)

• 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.

  nxdtest!14 (merged)

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

/cc @tomo, @jerome