fuzzer: add testonly to fuzzer tests

This will be needed to make some changes to `fuzzer_test`
(crrev.com/c/5053732), but also makes sense generally since those are
only meant to exist in tests, hence for testing.

Bug: chromium:1504840
Change-Id: Ic40178c96753179f485c90abb958874320192a3e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/328440
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Paul Semel <paulsemel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#41231}

test/fuzzers/BUILD.gn

@@ -12,6 +12,7 @@ import("../../webrtc.gni")
 
 rtc_library("webrtc_fuzzer_main") {
   sources = [ "webrtc_fuzzer_main.cc" ]
+  testonly = true
   deps = [
     "../../rtc_base:logging",
     "//testing/libfuzzer:fuzzing_engine_main",