You can always create a new library and place it just under the library that holds the APPLICATION, which is always listed first in the Library List.
Library List:
appname.pbl
new_unit_testing.pbl <---- you created this and placed it here in theLibrary List
app_lib1.pbl
app_lib2.pbl
app_lib3.pbl
app_lib4.pbl
.
.
.
Place your new code in NEW_UNIT_TESTING.PBL and it will take precedence over any object that has the same name in any library below it in the Library List.