Tests: Add regression tests for the LibCpp preprocessor

Similarly to the LibCpp parser regression tests, these tests run the
preprocessor on the .cpp test files under
Userland/LibCpp/Tests/preprocessor, and compare the output with existing
.txt ground truth files.

Tests/LibCpp/test-cpp-preprocessor.cpp

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2021, Itamar S. <itamar8910@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/LexicalPath.h>
+#include <LibCore/DirIterator.h>
+#include <LibCore/File.h>
+#include <LibCpp/Parser.h>
+#include <LibTest/TestCase.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+constexpr char TESTS_ROOT_DIR[] = "/home/anon/cpp-tests/preprocessor";
+
+static String read_all(const String& path)
+{
+    auto result = Core::File::open(path, Core::OpenMode::ReadOnly);
+    VERIFY(!result.is_error());
+    auto content = result.value()->read_all();
+    return { reinterpret_cast<const char*>(content.data()), content.size() };
+}
+
+TEST_CASE(test_regression)
+{
+    Core::DirIterator directory_iterator(TESTS_ROOT_DIR, Core::DirIterator::Flags::SkipDots);
+
+    while (directory_iterator.has_next()) {
+        auto file_path = directory_iterator.next_full_path();
+
+        auto path = LexicalPath { file_path };
+        if (!path.has_extension(".cpp"))
+            continue;
+
+        outln("Checking {}...", path.basename());
+
+        auto ast_file_path = String::formatted("{}.txt", file_path.substring(0, file_path.length() - sizeof(".cpp") + 1));
+
+        auto source = read_all(file_path);
+        auto target = read_all(ast_file_path);
+
+        StringView source_view(source);
+        Cpp::Preprocessor preprocessor(file_path, source_view);
+
+        auto target_lines = target.split_view('\n');
+        auto tokens = preprocessor.process_and_lex();
+
+        EXPECT_EQ(tokens.size(), target_lines.size());
+        for (size_t i = 0; i < tokens.size(); ++i) {
+            EXPECT_EQ(tokens[i].to_string(), target_lines[i]);
+        }
+    }
+}