[1.22] Terminate tests with SIGALRM if they take more than 120 seconds

The Debian autobuilders run cogl's build-time tests to get an idea of how much can be expected to work on each supported architecture. We're currently ignoring test failures and running the tests for information only, but if a test hangs, that will still cause the build to fail (and tie up an autobuilder until a rather long timeout is reached).

The fork of cogl in Mutter uses a 120 second timeout for tests in its Meson build system, but other than that it's purely an arbitrary number. dbus uses a similar alarm()-based watchdog, as do a couple of tests in GLib.

Bug-Debian: https://bugs.debian.org/935592