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/fn_inserter_test.cpp

@@ -0,0 +1,86 @@
+// 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/fn_inserter.h"
+
+#include "migrate_cpp/cpp_refactoring/matcher_test_base.h"
+
+namespace Carbon {
+namespace {
+
+class FnInserterTest : public MatcherTestBase {
+ protected:
+  FnInserterTest() : fn_inserter(replacements, &finder) {}
+
+  Carbon::FnInserter fn_inserter;
+};
+
+TEST_F(FnInserterTest, TrailingReturn) {
+  constexpr char Before[] = "auto A() -> int;";
+  constexpr char After[] = "fn A() -> int;";
+  ExpectReplacement(Before, After);
+}
+
+TEST_F(FnInserterTest, Inline) {
+  // TODO: Need to re-lex tokens, this should probably be "fn inline" for now.
+  constexpr char Before[] = "inline auto A() -> int;";
+  constexpr char After[] = "fn auto A() -> int;";
+  ExpectReplacement(Before, After);
+}
+
+TEST_F(FnInserterTest, Void) {
+  // TODO: void needs to be handled.
+  constexpr char Before[] = "void A();";
+  ExpectReplacement(Before, Before);
+}
+
+TEST_F(FnInserterTest, Methods) {
+  // TODO: void needs to be handled.
+  // TODO: Need to re-lex tokens, this should probably be "fn virtual" for now.
+  constexpr char Before[] = R"cpp(
+    class Shape {
+     public:
+      virtual void Draw() = 0;
+      virtual auto NumSides() -> int = 0;
+    };
+
+    class Circle : public Shape {
+     public:
+      void Draw() override;
+      auto NumSides() -> int override;
+      auto Radius() -> double { return radius_; }
+
+     private:
+      double radius_;
+    };
+  )cpp";
+  constexpr char After[] = R"(
+    class Shape {
+     public:
+      virtual void Draw() = 0;
+      fn auto NumSides() -> int = 0;
+    };
+
+    class Circle : public Shape {
+     public:
+      void Draw() override;
+      fn NumSides() -> int override;
+      fn Radius() -> double { return radius_; }
+
+     private:
+      double radius_;
+    };
+  )";
+  ExpectReplacement(Before, After);
+}
+
+TEST_F(FnInserterTest, LegacyReturn) {
+  // Code should be migrated to trailing returns by clang-tidy, so this is okay
+  // to miss.
+  constexpr char Before[] = "int A();";
+  ExpectReplacement(Before, Before);
+}
+
+}  // namespace
+}  // namespace Carbon