tests/qapi-schema: Tweak to demonstrate buggy member name check

tests/qapi-schema: Tweak to demonstrate buggy member name check

Member name 'u' and names starting with 'has-' or 'has_' are reserved
for the generator.  check_type() enforces this, covered by tests
reserved-member-u and reserved-member-has.

These tests neglect to cover optional members, where the name starts
with '*'.  Tweak reserved-member-u to fix that.  Test
reserved-member-has still covers non-optional members.

This demonstrates the reserved member name check is broken for
optional members.  The next commit will fix it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-7-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
[Commit message improved slightly]