Perl 5.36 has reclassified the warning condition that this test
case used, so that the expected error fails to appear.  Tweak
the test so it instead exercises a case that's handled the same
way in all Perl versions of interest.

This appears to meet our standards for back-patching into
out-of-support branches: it changes no user-visible behavior
but enables testing of old branches with newer tools.
Hence, back-patch as far as 9.2.

Dagfinn Ilmari Mannsåker, per report from Jitka Plesníková.

Discussion: https://postgr.es/m/564579.1654093326@sss.pgh.pa.us