Skip system-mode tests (only) if /var/tmp doesn't support xattrs

This partially reverts commit a53a752a35.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f