LibCpp: Add regression tests for the parser

For each .cpp file in the test suite data, there is a .ast file that
represents the "known good" baseline of the parser result.

Each .cpp file goes through the parser, and the result of
invoking `ASTNode::dump()` on the root node is compared to the
baseline to find regressions.

We also check that there were no parser errors when parsing the .cpp
files.

Userland/Libraries/LibCpp/Tests/function-decl.ast

@@ -0,0 +1,10 @@
+TranslationUnit[0:0->2:0]
+  FunctionDeclaration[0:0->2:0]
+    Type[0:0->0:4]
+      int
+    foo
+    (
+    )
+    FunctionDefinition[1:0->2:0]
+    {
+    }