tools: fix vm.run(user="root") on template imported from R4.2 (cont)

[marmarek]

Template imported from R4.2 already has qubes.VMRootShell service, but
it isn't really running as root by default. It's only R4.3+ when both
qubes.VMRootShell and qubes.VMRootExec got also force-user=root setting.
Since the check for VMRootExec is already needed anyway, use the same to
check if using qubes.VMRootShell makes sense, as both changes were
introduced at the same time in core-agent-linux:

1c538af2 Use rpc-config for running qubes.VMRoot* services as root
563a9cfa Add qubes.VMRootExec service

This is more flexible method than hardcoding version check.

And add a test for this case.

Fixes: f38c1d4 "use qubes.VMRootExec/qubes.VMRootShell services when root needed"
Fixes QubesOS/qubes-issues#10590

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.67%. Comparing base (38c79ef) to head (626633e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #467      +/-   ##
==========================================
+ Coverage   76.65%   76.67%   +0.02%     
==========================================
  Files          53       53              
  Lines        9407     9407              
==========================================
+ Hits         7211     7213       +2     
+ Misses       2196     2194       -2     

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