Force terminal width when running tests (pytest-dev#11425)

Related to pytest-dev#11423

(cherry picked from commit 241f2a8)

testing/conftest.py

@@ -21,6 +21,15 @@ def restore_tracing():
             sys.settrace(orig_trace)
 
 
+@pytest.fixture(autouse=True)
+def set_column_width(monkeypatch: pytest.MonkeyPatch) -> None:
+    """
+    Force terminal width to 80: some tests check the formatting of --help, which is sensible
+    to terminal width.
+    """
+    monkeypatch.setenv("COLUMNS", "80")
+
+
 @pytest.hookimpl(hookwrapper=True, tryfirst=True)
 def pytest_collection_modifyitems(items):
     """Prefer faster tests.