Add testing.md to project GEMINI.md

Our AI agents have had trouble finding the right binary to compile
and run when modifying tests, possibly because our gn targets are
non-standard.

Giving them access to g3doc/testing.md might help.

Bug: None
No-try: true
Change-Id: I981c4eb7ec002c494f581bc0ff55d67d847c6a30
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475920
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47819}