Fix test that expected "unknown hook" error on stderr

Author: nicoddemus

testing/test_helpconfig.py

@@ -33,7 +33,7 @@ def pytest_hello(xyz):
     """)
     result = testdir.runpytest()
     assert result.ret != 0
-    result.stderr.fnmatch_lines([
+    result.stdout.fnmatch_lines([
         '*unknown hook*pytest_hello*'
     ])