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.cpp

@@ -0,0 +1,38 @@
+// 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
+
+#include "migrate_cpp/cpp_refactoring/matcher_test_base.h"
+
+#include "clang/Tooling/Tooling.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace ct = ::clang::tooling;
+
+namespace Carbon {
+
+void MatcherTestBase::ExpectReplacement(llvm::StringRef before,
+                                        llvm::StringRef after) {
+  auto factory = ct::newFrontendActionFactory(&finder);
+  constexpr char Filename[] = "test.cc";
+  ASSERT_TRUE(ct::runToolOnCodeWithArgs(
+      factory->create(), before, {}, Filename, "clang-tool",
+      std::make_shared<clang::PCHContainerOperations>(),
+      ct::FileContentMappings()));
+  auto it = replacements.find(Filename);
+  if (it != replacements.end()) {
+    auto actual = ct::applyAllReplacements(before, it->second);
+    // Split lines to get gmock to get an easier-to-read error.
+    llvm::SmallVector<llvm::StringRef, 0> actual_lines;
+    llvm::SplitString(*actual, actual_lines, "\n");
+    llvm::SmallVector<llvm::StringRef, 0> after_lines;
+    llvm::SplitString(after, after_lines, "\n");
+    EXPECT_THAT(actual_lines, testing::ContainerEq(after_lines));
+  } else {
+    // No replacements; before and after should match.
+    EXPECT_THAT(before, testing::Eq(after));
+  }
+}
+
+}  // namespace Carbon