Use correct LLVM gtest/gmock for inference tests

Define a EqualsProto matcher that's missing from public gmock

PiperOrigin-RevId: 543694206
diff --git a/nullability/BUILD b/nullability/BUILD
index 8fc6c50..55e37e0 100644
--- a/nullability/BUILD
+++ b/nullability/BUILD
@@ -147,3 +147,16 @@
         "@llvm-project//third-party/unittest:gtest_main",
     ],
 )
+
+cc_library(
+    name = "proto_matchers",
+    testonly = 1,
+    srcs = ["proto_matchers.cc"],
+    hdrs = ["proto_matchers.h"],
+    visibility = [":__subpackages__"],
+    deps = [
+        "//third_party/protobuf",
+        "@llvm-project//llvm:Support",
+        "@llvm-project//third-party/unittest:gmock",
+    ],
+)