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/if-else.ast

@@ -0,0 +1,33 @@
+TranslationUnit[0:0->7:0]
+  FunctionDeclaration[0:0->7:0]
+    Type[0:0->0:4]
+      int
+    foo
+    (
+    )
+    FunctionDefinition[1:0->7:0]
+    {
+      IfStatement[2:4->3:16]
+        Predicate:
+        NumericLiteral[2:8->2:8]
+        2
+        Then:
+        ReturnStatement[3:8->3:16]
+          NumericLiteral[3:15->3:15]
+          2
+      IfStatement[4:4->5:16]
+        Predicate:
+        BinaryExpression[4:8->4:12]
+          NumericLiteral[4:8->4:8]
+          1
+          <
+          NumericLiteral[4:12->4:12]
+          2
+        Then:
+        ReturnStatement[5:8->5:16]
+          NumericLiteral[5:15->5:15]
+          1
+      ReturnStatement[6:4->6:12]
+        NumericLiteral[6:11->6:11]
+        0
+    }