tests: Factor out additional env vars mechanism

Make the mechanism more general, so that other tests can now specify
overrides for various defaults, if needed.

This replicates what GLib does:
https://gitlab.gnome.org/GNOME/glib/-/blob/main/glib/tests/meson.build#L475.

It introduces no functional changes in how the current set of tests run.

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

tests/meson.build

@@ -19,29 +19,30 @@ unit_testenv.append('GI_TYPELIB_PATH', gvc_typelib_path, separator: ':')
 unit_testenv.append('GI_TYPELIB_PATH', shell_typelib_path, separator: ':')
 unit_testenv.append('GI_TYPELIB_PATH', st_typelib_path, separator: ':')
 
-unit_tests = [
-    'breakManager',
-    'extensionUtils',
-    'highlighter',
-    'injectionManager',
-    'insertSorted',
-    'jsParse',
-    'markup',
-    'params',
-    'signalTracker',
-    'timeLimitsManager',
-    'url',
-    'versionCompare',
-]
+unit_tests = {
+  'breakManager': {},
+  'extensionUtils': {},
+  'highlighter': {},
+  'injectionManager': {},
+  'insertSorted': {},
+  'jsParse': {},
+  'markup': {},
+  'params': {},
+  'signalTracker': {},
+  'timeLimitsManager': {
+    # The time limits tests hard-code UTC to keep the code brief, so must be
+    # run in UTC
+    'env': { 'TZ': 'UTC' },
+  },
+  'url': {},
+  'versionCompare': {},
+}
 
-foreach test : unit_tests
+foreach test, extra_args : 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
+  foreach var, value : extra_args.get('env', {})
+    local_test_env.append(var, value)
+  endforeach
 
   test(test, jasmine,
     args: [