A bit of trickiness here.  The high points:

    - We need to feed --with-compiler to cabal.config, NOT
      the flag, because sandbox doesn't know how to read in
      compilers except from the global config.  This is
      arguably a bug but I can't be bothered to fix it.

    - We need to NOT provide certain commands when we're
      in sandbox mode; this is controlled by testHaveSandbox.

    - Use withSandbox to start a session in the sandbox.
      It inits the sandbox for you.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>