fixtures: make sure to properly setup stack for _fill_fixtures_impl

This code is weird, dead, deprecated and will be removed in pytest 7,
but for now some tests execute it, so fix it up in preparation for
some changes.

src/_pytest/fixtures.py

@@ -372,6 +372,7 @@ def _fill_fixtures_impl(function: "Function") -> None:
         fi = fm.getfixtureinfo(function.parent, function.obj, None)
         function._fixtureinfo = fi
         request = function._request = FixtureRequest(function, _ispytest=True)
+        fm.session._setupstate.prepare(function)
         request._fillfixtures()
         # Prune out funcargs for jstests.
         newfuncargs = {}