[1.22] tests: Use tmp file to dump test results
When running installed tests, the working directory for Cogl tests is in /usr/libexec/installed-tests, 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.
(Originally commit 05ab8eebe8f37d437fb915a2dafe0c0bb68e3950 in mutter's fork of cogl.)
Written by @feaneron, originally mutter@05ab8eeb.
Edited by Simon McVittie