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/matcher_test_base.h

@@ -0,0 +1,27 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef MIGRATE_CPP_CPP_REFACTORING_MATCHER_TEST_BASE_H_
+#define MIGRATE_CPP_CPP_REFACTORING_MATCHER_TEST_BASE_H_
+
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Tooling/Core/Replacement.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace Carbon {
+
+class MatcherTestBase : public ::testing::Test {
+ protected:
+  // Expects that that the replacements produced by running the finder result in
+  // the specified code transformation.
+  void ExpectReplacement(llvm::StringRef before, llvm::StringRef after);
+
+  std::map<std::string, clang::tooling::Replacements> replacements;
+  clang::ast_matchers::MatchFinder finder;
+};
+
+}  // namespace Carbon
+
+#endif  // MIGRATE_CPP_CPP_REFACTORING_MATCHER_TEST_BASE_H_