Improve wxTextCtrl::GetNumberOfLines() unit test and documentation.

Improve wxTextCtrl::GetNumberOfLines() unit test and documentation.

First, enable execution of the Lines() wxTextCtrl unit test case under wxOSX
as it passes since the fix in the previous commit.

Do add a test for GetNumberOfLines() that currently doesn't work in the same
way under all platforms -- but with this test we at least can be sure how does
it work exactly where.

Also mention the current discrepancy in this function behaviour in the
documentation.

See #12366.

Fix Lines() wxTextCtrl unit test to pass under wxGTK too.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68465 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775