Support for Single Mock Directory

Made a minor change to the makefile runner that allows the test's makefile to specify specific mock files to compile into the test (rather than complete directories).  

My motivation was to cover the case that all mocks live within a single directory and can be pulled in selectively.  This was particularly important in the case where I had two modules dependent on each other that were being tested separately.  If the full mock directory got pulled in by the makefile, the under test and mock implementations would both get compiled.