tests: Fix gdatetime test on non-UTC systems
Commit 0b114b26 accidentally made the
tests only pass on systems running in a UTC timezone, as it checked the
local timezone %Z format against UTC.
This happens to pass on all the GLib CI runners except the macOS one, which is running under CET.
Make the formatting tests timezone-independent by running them with UTC
datetimes. There’s already a separate test for checking that %Z works
correctly in a non-UTC timezone (test_z).
Signed-off-by: Philip Withnall pwithnall@endlessos.org