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/struct.ast

@@ -0,0 +1,41 @@
+TranslationUnit[1:0->12:0]
+  StructOrClassDeclaration[1:0->7:0]
+  MyStruct
+    MemberDeclaration[3:4->4:4]
+      Type[3:4->3:8]
+        int
+      x
+    MemberDeclaration[4:4->5:0]
+      Pointer[4:12->4:14]
+        Type[4:4->4:12]
+          s
+      next
+  FunctionDeclaration[7:0->12:0]
+    Type[7:0->7:4]
+      int
+    foo
+    (
+    )
+    FunctionDefinition[8:0->12:0]
+    {
+      VariableDeclaration[9:4->9:14]
+        Type[9:4->9:13]
+          MyStruct
+        s
+      FunctionCall[10:4->10:23]
+        Name[10:4->10:10]
+        printf
+        StringLiteral[10:11->10:16]
+          "%d\n"
+        MemberExpression[10:19->10:22]
+          Name[10:19->10:20]
+          s
+          Identifier[10:21->10:21]
+          x
+      ReturnStatement[11:4->11:14]
+        MemberExpression[11:11->11:14]
+          Name[11:11->11:12]
+          s
+          Identifier[11:13->11:13]
+          x
+    }