Add tests to the cpp_refactoring tool. (#539)

This refactors the main.cpp out into a file structure that should make it easier to add more matchers/tests.

migrate_cpp/cpp_refactoring/BUILD

@@ -2,7 +2,7 @@
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-load("@rules_cc//cc:defs.bzl", "cc_binary")
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -10,7 +10,48 @@ cc_binary(
     name = "cpp_refactoring",
     srcs = ["main.cpp"],
     deps = [
-        "@llvm-project//clang:ast_matchers",
+        ":fn_inserter",
         "@llvm-project//clang:tooling",
     ],
 )
+
+cc_library(
+    name = "fn_inserter",
+    srcs = ["fn_inserter.cpp"],
+    hdrs = ["fn_inserter.h"],
+    deps = [":matcher"],
+)
+
+cc_test(
+    name = "fn_inserter_test",
+    srcs = ["fn_inserter_test.cpp"],
+    deps = [
+        ":fn_inserter",
+        ":matcher_test_base",
+        "@llvm-project//clang:tooling",
+        "@llvm-project//llvm:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "matcher",
+    srcs = ["matcher.cpp"],
+    hdrs = ["matcher.h"],
+    deps = [
+        "@llvm-project//clang:ast_matchers",
+        "@llvm-project//clang:tooling_core",
+    ],
+)
+
+cc_library(
+    name = "matcher_test_base",
+    testonly = 1,
+    srcs = ["matcher_test_base.cpp"],
+    hdrs = ["matcher_test_base.h"],
+    deps = [
+        "@llvm-project//clang:ast_matchers",
+        "@llvm-project//clang:tooling",
+        "@llvm-project//llvm:gmock",
+        "@llvm-project//llvm:gtest",
+    ],
+)