fixtures: make FixtureLookupError work from TopRequest with custom message

In this case let's just show the test function.

Author: bluetech

src/_pytest/fixtures.py

@@ -808,16 +808,7 @@ def formatrepr(self) -> FixtureLookupErrorRepr:
         stack = [self.request._pyfuncitem.obj]
         stack.extend(map(lambda x: x.func, self.fixturestack))
         msg = self.msg
-        # This function currently makes an assumption that a non-None msg means we
-        # have a non-empty `self.fixturestack`. This is currently true, but if
-        # somebody at some point want to extend the use of FixtureLookupError to
-        # new cases it might break.
-        # Add the assert to make it clearer to developer that this will fail, otherwise
-        # it crashes because `fspath` does not get set due to `stack` being empty.
-        assert self.msg is None or self.fixturestack, (
-            "formatrepr assumptions broken, rewrite it to handle it"
-        )
-        if msg is not None:
+        if msg is not None and len(stack) > 1:
             # The last fixture raise an error, let's present
             # it at the requesting side.
             stack = stack[:-1]