Unit tests for strncmp_iw_with_mode

Unit tests for strncmp_iw_with_mode

This patch attempts to make a start at adding unit tests for
strncmp_iw_with_mode.  While there is quite a bit of testing
of this function in other tests, these are currently end-to-end
tests.

This patch attempts to cover the basics of string matching, white
space, C++ ABI tags, and several other topics. However, one area
that is ostensibly missing is testing the `match_for_lcd' feature.
This is otherwise tested as part of our end-to-end DejaGNU-based
testing.