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.cpp

@@ -0,0 +1,8 @@
+int foo()
+{
+    if (2)
+        return 2;
+    if (1 < 2)
+        return 1;
+    return 0;
+}