Fix downstream gating tests on RHEL 9

[martinpitt]

There are a lot of failures in current RHEL 9, most of them rather weird:

# testDetachDisk (__main__.TestMachinesDisks)
Traceback (most recent call last):
  File "/usr/lib64/python3.9/unittest/case.py", line 550, in _callTestMethod
    method()
  File "/ARTIFACTS/work-upstreamF2UGhr/plans/upstream/discover/default/tests/test/check-machines-disks", line 810, in testDetachDisk
    m.execute("virsh attach-disk --domain subVmTest1 --source {0}/mydiskofpoolone_2 --target vdd --targetbus virtio --persistent".format(p1))
  File "/ARTIFACTS/work-upstreamF2UGhr/plans/upstream/discover/default/tests/bots/machine/machine_core/ssh_connection.py", line 322, in execute
    ret = select.select(rset, wset, [], 10)
  File "/ARTIFACTS/work-upstreamF2UGhr/plans/upstream/discover/default/tests/bots/machine/machine_core/timeout.py", line 41, in handle_timeout
    raise RuntimeError(self.error_message)
RuntimeError: Timed out on 'virsh attach-disk --domain subVmTest1 --source /var/lib/cockpittest/vm_one/mydiskofpoolone_2 --target vdd --targetbus virtio --persistent'
[0422/092906.620137:WARNING:headless_browser_main_parts.cc(83)] Cannot create Pref Service with no user data dir.

DevTools listening on ws://127.0.0.1:9430/devtools/browser/ba9c75fc-dd2c-4720-a11b-9ed54c52208e
/ARTIFACTS/work-upstreamF2UGhr/plans/upstream/discover/default/tests/test/common/chromium-cdp-driver.js:242
        throw Error(`Frame ${frame} is unknown`);

I can actually reproduce this with

bots/vm-run rhel-9-0
tmt run --all provision --how connect -g c

but I can't make much sense of it. I did see that the memory gets really tight. I propose to go with this reduced test set for now, and slowly build this back up.

There is also a few additional commits which fix the test setup for the above "run this locally" scenario.

[martinpitt]

I did a downstream test run and it succeeded 🥳 (direct TF log)

Our own tests fail due to testNICEdit, issue #127 .