test: add test-lib.el file with `visible-buffer-string' function

The patch adds test-lib.el file for Emacs tests auxiliary stuff.
Currently, it implements two functions: `visible-buffer-string'
and `visible-buffer-substring'.  These are similar to standard
counterparts without "visible-" prefix but exclude invisible
text.  The functions are not used anywhere at the moment but
should be useful for testing hiding/showing in the Emacs
interface.

Edited-by: Carl Worth <cworth@cworth.org> Fixed "basic" test to ignore
new test-lib.el file.