tests: fix a typo in test-demandimport.t stable

tests: fix a typo in test-demandimport.t

I guess we figured out why it wasn't lazy.

--- a/tests/test-demandimport.py	Tue Jan 10 17:23:13 2023 +0400
+++ b/tests/test-demandimport.py	Tue Jan 10 17:35:53 2023 +0400
@@ -82,8 +82,7 @@
 from mercurial import error as errorproxy
 
 if ispy3:
-    # unsure why this isn't lazy.
-    assert not isinstance(f, _LazyModule)
+    assert isinstance(errorproxy, _LazyModule)
     assert f(errorproxy) == "<module 'mercurial.error' from '?'>", f(errorproxy)
 else:
     assert f(errorproxy) == "<unloaded module 'error'>", f(errorproxy)