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.

Userland/Libraries/LibCpp/Tests/preprocessor/macro1.cpp

@@ -0,0 +1,2 @@
+#define ADD(x,y) x+y
+ADD(2,5);