Minor test fixes in 035_standby_logical_decoding.pl

Minor test fixes in 035_standby_logical_decoding.pl

Import usleep, which, due to an oversight in oversight in commit
48796a98d5ae was used but not imported.

Correct the comparison string used in two logfile checks. Previously, it
was incorrect and thus the test could never have failed.

Also wordsmith a comment to make it clear when hot_standby_feedback is
meant to be on during the test scenarios.

Reported-by: Melanie Plageman <melanieplageman@gmail.com>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_YO2mEm%3DZWZKPjTMU%3DgW5Y83_KMi_1cr51JwavH0ctd7w%40mail.gmail.com
Backpatch-through: 16