In some setups (eg. macports) $tmpdir can expand to more than
100 symbols and tests fail with 'socket file name too long'
errors.
Using relative path to socket file fixes the issue.
The tmux-prompt test was failing when run more than once, because
XDG_DATA_HOME has a leading double-dot, causing the uvars file to
leak across sessions. Descend more deeply into our tmpdir to isolate
our XDG_DATA_HOME.
This reverts commit b56b230076.
which somehow made us miss repaints on uvar notifications.
The commit was a workaround for a polling bug which was later properly
fixed by 7c5b8b855 ("Use the uvar notifier pipe timestamp to avoid
excessive polling"), so it's no longer necessary.
Add a system test. If I had a better understanding of the bug I could
probably write a better test.
Fixes#8088
