Add gmock matcher support for comparing flatbuffers

This makes it easier to compare flatbuffers in tests.

Change-Id: Iabd1038f7ce9dfc3edf1162f2d7b0485cdda3b90
diff --git a/aos/testing/BUILD b/aos/testing/BUILD
index 116acc0..d3287e3 100644
--- a/aos/testing/BUILD
+++ b/aos/testing/BUILD
@@ -102,3 +102,18 @@
     target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
+
+cc_library(
+    name = "flatbuffer_eq",
+    testonly = True,
+    hdrs = [
+        "flatbuffer_eq.h",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//aos:flatbuffer_merge",
+        "//aos:flatbuffers",
+        "//aos:json_to_flatbuffer",
+        "@com_google_googletest//:gtest",
+    ],
+)