tests: Add variable expansion tests

There are some corner cases in variable expansion which would be nice to
capture before going and rewriting the variable expansion code. The
majority of these are related to configuring files and strings with '@'
in them in conjunction with @ONLY being specified. Another is testing
for '(' usage inside of ENV variable references based on whether it is
quoted or not.
CPackWIX: refactor and cleanup

Extract addtional classes and functions which are getting unsightly large.
Use some of the coding conventions more consistently.
Merge topic 'wix-refactor' into next

ce369672 CPackWIX: refactor and cleanup
23b6df89 tests: Add variable expansion tests