For test apexes, base_apex_name is the api domain

apex_test do not "override" the source apexes that they test. However,
similar to override_apexes, test apexes have the same path
(/apex/<apex_name>) on device. Instead of creating another property,
reuse the existing base_apex_name property. The use case for this will
be for creating selection statements for stub/impl selection.

Test: go test ./bp2build
Change-Id: I4b7548e0e0fc920e407e1c5e5c00e87efc6e70c9