Mention all variables used in surgsim_add_unit_tests
Mention all variables used in surgsim_add_unit_tests

Readd mention of UNIT_TEST_SHARED_LIBS, UNIT_TEST_SHARED_RELEASE_LIBS,
and UNIT_TEST_SHARED_DEBUG_LIBS in the comment of
surgsim_add_unit_tests. This was lost in the merging of the unit test
macros into one macro.

simquest/opensurgsim,simquest/opensurgsim,simquest/opensurgsim,simquest/opensurgsim