tests/u, tests/i: set TZ variable before running tests

* We were using the pytest-env plugin to run the tests in a non-UTC time
  zone.
* The pytest-env plugin doesn't work with pytest-xdist so this was being
  ignored.
* Also due to the way TZ support works in Python, changing the env var
  whilst Python is running may or may not result in changes.

.github/workflows/test_fast.yml

@@ -24,8 +24,12 @@ jobs:
         include:
           - os: 'macos-latest'
             python-version: '3.7'
+
     env:
+      # Use non-UTC time zone
+      TZ: XXX-09:35
       PYTEST_ADDOPTS: --cov --cov-append -n 5 --color=yes
+
     steps:
       - name: Checkout
         uses: actions/checkout@v4

pytest.ini

@@ -32,13 +32,10 @@ testpaths =
     cylc/flow/
     tests/unit/
     tests/integration/
-env =
-    # a weird timezone to check that tests aren't assuming the local timezone
-    TZ=XXX-09:35
 doctest_optionflags =
     NORMALIZE_WHITESPACE
     IGNORE_EXCEPTION_DETAIL
     ELLIPSIS
 asyncio_mode = auto
 markers=
-    linkcheck: Test links
+    linkcheck: Test links

setup.cfg

@@ -121,7 +121,6 @@ tests =
     pytest-asyncio>=0.17,!=0.23.*
     pytest-cov>=2.8.0
     pytest-xdist>=2
-    pytest-env>=0.6.2
     pytest>=6
     testfixtures>=6.11.0
     towncrier>=23