When running installed tests, the working directory for Cogl
tests is /usr/libexec/installed-tests/mutter-cogl-4/conform,
which isn't writable by normal users.
To avoid the adding stray hidden files to the current directory,
adapt the runner script to fallback to $(mktemp) - which is
available on all platform we care about - and avoid adding
hidden files everywhere.
05ab8eebe8