Add a test that all //api/test headers are compilable.

Bug: webrtc:10376
Change-Id: I2a1ea24ddf5980c76660724fae68c16179bb25a7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/125682
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Yves Gerey <yvesg@google.com>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27119}
diff --git a/BUILD.gn b/BUILD.gn
index ae2f716..861644d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -474,6 +474,7 @@
 
     deps = [
       ":webrtc_common",
+      "api:compile_all_headers",
       "api:rtc_api_unittests",
       "api/audio/test:audio_api_unittests",
       "api/audio_codecs/test:audio_codecs_api_unittests",