* Keep BUILD_TESTING, but only run tests compatible with API (for
packaging)
* Add BUILD_TESTING_INTERNAL for all tests including internal function
tests that modify API to be run on our CI
Building client channels out of tree does not work reliable as the
dependencies can not be easily split. (exceptions may be simple ones
as echo channel)
It does complicate the build system and code though, so remove this
for maintainability reasons.
