Improve testing of custom derive support for data-carrying enums

[joshlf]

Following up on #879.

Progress

• Write tests which ensure that a malicious use can't introduce variables which are not shadowed by our derive-generated code and cause problems
  • Migrated to #1684 since this doesn't need to block 0.8
• Write tests which assert the token stream output of the derives
  • Done: [derive] Test TryFromBytes output on dataful enums #1665
• https://github.com/google/zerocopy/pull/1665/files#r1761803556
  • [derive] More complex TryFromBytes enum output tests #1683