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.

Meta/lint-clang-format.sh

@@ -13,7 +13,8 @@ if [ "$#" -eq "1" ]; then
             ':!:Base' \
             ':!:Kernel/FileSystem/ext2_fs.h' \
             ':!:Userland/DevTools/HackStudio/LanguageServers/Cpp/Tests/*' \
-            ':!:Userland/Libraries/LibCpp/Tests/parser/*'
+            ':!:Userland/Libraries/LibCpp/Tests/parser/*' \
+            ':!:Userland/Libraries/LibCpp/Tests/preprocessor/*'
     )
 else
     files=()