Propagate cancellation to spawned test jobs

So that if a test run is canceled in ERP5 UI, nxdtest stops its run
soon, instead of after several hours in case of
SlapOS.SoftwareReleases.IntegrationTest-* tests.

See the main (938b5455) and other patches for details.

/helped-and-reviewed-by @jerome
/reviewed-on !14

* y/cancel:
  Add test for cancel propagation
  tests: Run nxdtest.main for each test in a separate thread, so that pytest.timeout generally works
  Propagate cancellation to spawned test jobs
  Raise ctx.err() if test run was cancelled
  Stop spawned process softly on ctx cancel