Tracking rbe tools with a GN target

Files under the tools directory were untracked by GN.

This cl creates a target for them. The target is testonly because it
depends on "webrtc/test:rtp_test_utils".

BUG=webrtc:7651
NOTRY=True

Review-Url: https://codereview.webrtc.org/2886713002
Cr-Original-Commit-Position: refs/heads/master@{#19683}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2877eaf7fe2759b5464596ceca4e8f640f809a16