Add relative path support to test harness

Add relative path support to test harness

Makes sure the loader test framework can create relative paths in
the library_path fields. Adds a test to insure those paths still
work.

Since the contents of "library_path" is mututally exclusive with
the existing use_dynamic_library_default_search_paths, a new enum
was added to allow tests to choose which of the three content
types a given ICD or Layer manifest should use.