Resolve complex arguments to LESS helper functions

The icons in Flow and the Compact Personal Bar beta feature are missing,
their generated background-image 7 is empty.

The lessphp implementation passes full data frames into the helper
functions.  Utilize the now-public compileValue method of less
to evaluate the data frame into a string.

Additionally adds a small abstraction to run pairs of less/css files
as unit tests by executing the less file and comparing it to the
equivalent css file.

Change-Id: I1704f84638d86a0e6e6b9c190972ab19180bd484