[PR #13980/9f992b8b backport][9.0.x] config: Add a test for -o with invalid option (#13994)

* config: Add a test for -o with invalid option (#13980)

This was ignored until #13830. With that change, it now gets correctly surfaced
to the user as a warning (or error with --strict-config), so we should have a
test for it.

(cherry picked from commit 9f992b8)

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---------

Co-authored-by: Florian Bruhin <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

testing/test_config.py

@@ -2356,7 +2356,16 @@ def test_override_ini_without_config_file(self, pytester: Pytester) -> None:
             }
         )
         result = pytester.runpytest("--override-ini", "pythonpath=src")
-        assert result.parseoutcomes() == {"passed": 1}
+        result.assert_outcomes(passed=1)
+
+    def test_override_ini_invalid_option(self, pytester: Pytester) -> None:
+        result = pytester.runpytest("--override-ini", "doesnotexist=true")
+        result.stdout.fnmatch_lines(
+            [
+                "=*= warnings summary =*=",
+                "*PytestConfigWarning:*Unknown config option: doesnotexist",
+            ]
+        )
 
 
 def test_help_via_addopts(pytester: Pytester) -> None: