Simplify a FormattedExcinfo test

The previous test was better in that it used fakes to test all of the
real code paths. The problem with that is that it makes it impossible to
simplify the code with `isinstance` checks. So let's just simulate the
issue directly with a monkeypatch.

testing/code/test_excinfo.py

@@ -502,65 +502,18 @@ raise ValueError()
         assert repr.reprtraceback.reprentries[1].lines[0] == ">   ???"
         assert repr.chain[0][0].reprentries[1].lines[0] == ">   ???"
 
-    def test_repr_source_failing_fullsource(self):
+    def test_repr_source_failing_fullsource(self, monkeypatch) -> None:
         pr = FormattedExcinfo()
 
-        class FakeCode:
-            class raw:
-                co_filename = "?"
+        try:
+            1 / 0
+        except ZeroDivisionError:
+            excinfo = ExceptionInfo.from_current()
 
-            path = "?"
-            firstlineno = 5
+        with monkeypatch.context() as m:
+            m.setattr(_pytest._code.Code, "fullsource", property(lambda self: None))
+            repr = pr.repr_excinfo(excinfo)
 
-            def fullsource(self):
-                return None
-
-            fullsource = property(fullsource)
-
-        class FakeFrame:
-            code = FakeCode()
-            f_locals = {}
-            f_globals = {}
-
-        class FakeTracebackEntry(_pytest._code.Traceback.Entry):
-            def __init__(self, tb, excinfo=None):
-                self.lineno = 5 + 3
-
-            @property
-            def frame(self):
-                return FakeFrame()
-
-        class Traceback(_pytest._code.Traceback):
-            Entry = FakeTracebackEntry
-
-        class FakeExcinfo(_pytest._code.ExceptionInfo):
-            typename = "Foo"
-            value = Exception()
-
-            def __init__(self):
-                pass
-
-            def exconly(self, tryshort):
-                return "EXC"
-
-            def errisinstance(self, cls):
-                return False
-
-        excinfo = FakeExcinfo()
-
-        class FakeRawTB:
-            tb_next = None
-
-        tb = FakeRawTB()
-        excinfo.traceback = Traceback(tb)
-
-        fail = IOError()
-        repr = pr.repr_excinfo(excinfo)
-        assert repr.reprtraceback.reprentries[0].lines[0] == ">   ???"
-        assert repr.chain[0][0].reprentries[0].lines[0] == ">   ???"
-
-        fail = py.error.ENOENT  # noqa
-        repr = pr.repr_excinfo(excinfo)
         assert repr.reprtraceback.reprentries[0].lines[0] == ">   ???"
         assert repr.chain[0][0].reprentries[0].lines[0] == ">   ???"