bwe_test_logging.cc is supposed to be conditionally built in gyp builds
but, due to a path error in the sources! expressions it was always
compiled.
Meanwhile, compilation of bwe_test_logging.cc was never set up for gn
builds.
This fixes both of these problems.
BUG=604060
Review-Url: https://codereview.webrtc.org/1990373002
Cr-Commit-Position: refs/heads/master@{#12842}