Beware gb_CppunitTest_use_library_files plus gb_CppunitTest_use_component

...it causes the library's objects to be loaded twice, causing potential trouble
with static data.  So introduce gb_Cppunit_set_componentfile, processing the
library's source component file into an additional workdir/CppuntiTest/
component file referencing the test library.  (This required to drop the
gb_Helper_abbreviate_dirs bric-a-brac, as it would do $W replacement into file
URLs contained in the resulting .component file.)

Change-Id: I57847e549817d25cca53a13effd7d408e0c99f9a