gh-117381: Fix error message for ntpath.commonpath

[nineteendo]

• Issue: Fix error message for ntpath.commonpath #117381

[aisk]

I think maybe we need a test to ensure the expected error will be raised.

[nineteendo]

There are now assertRaisesRegex tests for all these cases:

Created with https://csacademy.com/app/graph_editor (mermaid can't connect horizontally):

D:/foo C:/foo
D:/foo C:foo
D:foo C:/foo
D:foo C:foo
C:/foo C:foo
C:/foo /foo
C:/foo foo
C:foo /foo
C:foo foo
/foo foo

[serhiy-storchaka]

LGTM.