qapi: Add tests of redefined expressions

Demonstrate that the qapi generator doesn't deal very well with
redefined expressions.  At the parse level, they are silently
accepted; and while the testsuite just stops at parsing, I've
further tested that many of them cause generator crashes or
invalid C code if they were appended to qapi-schema-test.json.
A later patch will tighten things up and adjust the testsuite
to match.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
