test: declare tests/_fixtures as a non-mirror directory

07b39f1e8a split tests/conftest.py into topic modules under
tests/_fixtures, but the layout invariant only admits directories that
mirror a source package or are declared in _NON_MIRROR_DIRS, so the
Python lane failed test_every_test_directory_mirrors_a_source_directory_
or_is_declared. The directory holds suite-wide conftest plumbing, not
tests; declare it with its reason.

tests/test_tests_tree_layout.py

@@ -26,8 +26,10 @@ TESTS_ROOT = REPO_ROOT / "tests"
 # "compat": frozen pre-PM updater import surface (tests/compat/README.md) — the
 # audited names live only in history, so there is no source package to mirror.
 # "installation": python driver for the tests/install shell e2e assets.
+# "_fixtures": topic modules tests/conftest.py is split into — suite-wide
+# plumbing (env filtering, live-system guard, platform gating), not tests.
 _NON_MIRROR_DIRS = {
-    "compat", "installation",
+    "compat", "installation", "_fixtures",
     "ci", "conformance", "dashboard", "desktop", "docker", "e2e", "evals",
     "fakes", "fixtures", "honcho_plugin", "install", "integration", "manual",
     "monitoring", "openviking_plugin", "perf_guards", "scripts", "secret_sources",