[Perl/perl5] efd192: Use strict and warnings in B::Deparse tests

[Dagfinn Ilmari Mannsåker via perl5-changes]

  Branch: refs/heads/deparse-tests-warnings
  Home:   https://github.com/Perl/perl5
  Commit: efd192f2f304b7e7b9ff6971c7f195688eeeed02
      
https://github.com/Perl/perl5/commit/efd192f2f304b7e7b9ff6971c7f195688eeeed02
  Author: Dagfinn Ilmari Mannsåker <ilm...@ilmari.org>
  Date:   2023-07-20 (Thu, 20 Jul 2023)

  Changed paths:
    M lib/B/Deparse-core.t
    M lib/B/Deparse-subclass.t

  Log Message:
  -----------
  Use strict and warnings in B::Deparse tests

This would have caught the duplicate (and it turns out, useless)
definition of $expr in do_infix_keyword().