tests/functional2/lang: Refactor test outputs

How long do we have the value printer already? It's time to stop
concatenating strings like it's 2005

Change-Id: I3f5074de2439a1ad78af94de877bb141bc9f1d82

tests/functional2/lang/string/in.nix

@@ -1,12 +1,14 @@
-"foo" + "bar"
-  + toString (/a/b + /c/d)
-  + toString (/foo/bar + "/../xyzzy/." + "/foo.txt")
-  + ("/../foo" + toString /x/y)
-  + "escape: \"quote\" \n \\"
-  + "end
+[
+  ("foo" + "bar")
+  (toString (/a/b + /c/d))
+  (toString (/foo/bar + "/../xyzzy/." + "/foo.txt"))
+  ("/../foo" + toString /x/y)
+  "escape: \"quote\" \n \\"
+  "end
 of
 line"
-  + "foo${if true then "b${"a" + "r"}" else "xyzzy"}blaat"
-  + "foo$bar"
-  + "$\"$\""
-  + "$"
+  "foo${if true then "b${"a" + "r"}" else "xyzzy"}blaat"
+  "foo$bar"
+  "$\"$\""
+  "$"
+]