tests can fail if a display is in use

[Antoine Martin]

If a vfb is left running from an aborted or failed test then we may try to start a new one on the same display number and fail.

Detected thanks to r20471. Somewhat related to #1963.

[Antoine Martin]

• r20472: don't fail if the socket is not removed, log a warning - may undo this one, sockets should always end up being removed!
• r20473: use a private xauth file that we cleanup afterwards, prevents relying on the file (not) existing and being in a good state (permissions, etc)
• r20475 bug fix for xauth paths

More test improvements: r20474, r20476, r20492, r20493, r20495

The tests now run without errors again, including on the new buildbot!

The new buildbot also uses nspawn to protect and better isolate the host from the build environment. So failures in one build "chroot" should not caused failures for other distros.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1965