test expressions in advanced

[Matthew Flatt <mflatt@cs.utah.edu>]

The DrScheme manual for v103 says that tests for `if', `cond', etc. can
return an arbitrary value.

The "PLT DrScheme: Advanced Student Language" manual says that tests
must be boolean-valued.

HtDP implicitly claims that tests must be boolean-valued (because it
implies that `cond' in Advanced works like `cond' in Intermediate).

The v103 implementation matches the DrScheme manual.

>>>>> I'm inclined to require boolean-valued tests when implementing
Advanced for v200. Any objections?

Matthew