lint: silence B018 in test_dunder_getattr_missing_raises

ruff 0.16 enables flake8-bugbear by default, so the bare ``pluggy.nope``
expression added in #718 fails B018 and turns pre-commit.ci red on main.
Bind it to ``_`` like the other attribute-access checks in that test suite.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: RonnyPfannschmidt

testing/test_details.py

@@ -236,7 +236,7 @@ def test_dunder_version() -> None:
 
 def test_dunder_getattr_missing_raises() -> None:
     with pytest.raises(AttributeError, match="module pluggy has no attribute 'nope'"):
-        pluggy.nope
+        _ = pluggy.nope
 
 
 def test_hookimpl_disallow_invalid_combination() -> None: