tests: Stop using deprecated g_test_trap_fork() API
Use g_test_subprocess() and g_test_trap_subprocess() instead. This
requires splitting up some of the unit tests, since subprocesses are
handled at a per-test level, and you can’t trivially fork multiple
different subprocesses from a single test (like you could with fork()).
While fork was safe to use on POSIX (and dconf only targets POSIX), the
fact that we had to redefine GLIB_VERSION_MIN_REQUIRED to hide the
deprecation errors was not doing wonders for the maintainability of the
tests.
Signed-off-by: Philip Withnall <withnall@endlessm.com>