export common bp2build testing functions

bp2buildTestCase
attrNameToString
runBp2BuildTestCase
makeBazelTargetNoRestrictions

The testing framework defined in the bp2build package can only be used
from within the package because many common testing functions are
private to the package. This prevents modules defined in Soong
plugins (e.g. system/tools/aidl/build) from testing bp2build conversions.

Test: go test ./bp2build
Change-Id: Ia867081327c5181d04687b13c4550e68e6a11f86