tests: Run timeLimitsManager tests in UTC

In order to keep the test code brief, all expected times and dates are
written as ISO 8601 strings in the test code, and use `Z` to specify UTC
(again, to keep things brief).

If the test is run under a significantly different timezone, the
end-of-day timestamp changes significantly, as it’s calculated by the
code-under-test using the local timezone.

Avoid that issue by requiring the `timeLimitsManager` tests to always be
run in UTC.

The previous failure could be reproduced by running `TZ=Europe/Madrid
meson test -C /opt/gnome/build/gnome-shell timeLimitsManager --verbose`
— the test for “tracks a single day’s usage early in the morning” would
fail.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3605>

tests/meson.build

@@ -35,6 +35,14 @@ unit_tests = [
 ]
 
 foreach test : unit_tests
+  local_test_env = unit_testenv
+
+  # The time limits tests hard-code UTC to keep the code brief, so must be run
+  # in UTC
+  if test == 'timeLimitsManager'
+    local_test_env.set('TZ', 'UTC')
+  endif
+
   test(test, jasmine,
     args: [
       '--module',
@@ -44,7 +52,7 @@ foreach test : unit_tests
       'unit/@0@.js'.format(test),
     ],
     suite: 'unit',
-    env: unit_testenv,
+    env: local_test_env,
     protocol: 'tap',
     workdir: meson.current_source_dir())
 endforeach