Ignore test apexes from bp2build generated tags

Soong does not enforce apex_available on the contents of test apex. To
prevent special-casing test apexes in the apex validation aspect in
Bazel, drop the test apexes from the tags altogether.

( The core problem I am trying to solve is making sure that stub
libraries in Bazel have a single apex available. apex validation happens
to be a nice side benefit)

Bug: 277651159
Test: go test ./bp2build
Change-Id: Ibb3cfedb5c0f2cda0464bf3758c70b67cb5885d1