Add test for extension enabling in parser

tests/cpp-2/Main.hs

@@ -0,0 +1,7 @@
+module Main where
+
+main :: IO ()
+main = putStrLn "hello"
+
+foo :: forall a. a -> a
+foo = id

tests/cpp-2/golden

@@ -0,0 +1 @@
+hello

tests/cpp-2/package.nix

@@ -0,0 +1,4 @@
+{ main = "Main";
+  src = ./.;
+  extensions = ["RankNTypes" "CPP"];
+}

tests/cpp-2/test

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# vim: ft=sh sw=2 et
+
+set -euo pipefail
+
+test() {
+  $SNACK build
+  $SNACK run | diff golden -
+
+  TMP_FILE=$(mktemp)
+
+  capture_io "$TMP_FILE" main | $SNACK ghci
+
+  diff golden $TMP_FILE
+  rm $TMP_FILE
+}
+
+SNACK="snack -j4" test