Bug 1637041: fix test_presets.py on windows r=rstewart

In some NT-specific code, a list of "items()" was being updated. In python 3, to modify the result
of "items()", you have to gain ownership first by explicitly converting it into a list().

Also resolves some flaky failures that were seen locally by marking test_presets.py as sequential.

Differential Revision: https://phabricator.services.mozilla.com/D74793

tools/tryselect/test/conftest.py

@@ -79,7 +79,7 @@ def pytest_generate_tests(metafunc):
     elif all(fixture in metafunc.fixturenames for fixture in ('shared_name', 'shared_preset')):
         preset_path = os.path.join(push.build.topsrcdir, 'tools', 'tryselect', 'try_presets.yml')
         with open(preset_path, 'r') as fh:
-            presets = yaml.safe_load(fh).items()
+            presets = list(yaml.safe_load(fh).items())
 
         ids = [p[0] for p in presets]
 

tools/tryselect/test/python.ini

@@ -7,5 +7,8 @@ requirements = tools/tryselect/selectors/chooser/requirements.txt
 [test_fuzzy.py]
 [test_mozharness_integration.py]
 [test_presets.py]
+# Modifies "task_duration_history.json" in .mozbuild. Since other tests depend on this file, this test
+# shouldn't be run in parallel with those other tests.
+sequential = true
 [test_tasks.py]
 [test_task_configs.py]