Make tests work in Racket 5.3.2

In Racket 5.3.2, the `expand` tests would error with:

    link: module mismatch;
     possibly, bytecode file needs re-compile because dependencies changed
      exporting module: "/Applications/Racket_v5.3.2/collects/racket/private/kw.rkt"
      exporting phase level: 0
      internal explanation: variable not provided (directly or indirectly)
      at: #%app
      in: new-app

Use another approach to check whether the default or custom #%app is
used.