Add new pending test for sbt/sbt#1237

This test shows that macros that simply return their argument produce
a stack overflow during extraction of used names.

Consider a macro `foo(c: Context)(a: c.Expr[Any]) = a`. An
application of this macro such as `foo(someVal)` will lead to the
expansion `someVal`. sbt will extract the `original` tree from it,
and find `foo(someVal)`, and recurse infinitely on `someVal`.