Fix src/test/subscription/t/029_on_error.pl test when wal_debug is enabled

Hello, hackers!

Recently I've been building postgres with different cflags and cppflags.
And suddenly on REL_15_STABLE, REL_16_STABLE and master
I faced a failure of a src/test/subscription/t/029_on_error.pl test when
      CPPFLAGS="-DWAL_DEBUG"
and
      printf "wal_debug = on\n" >> "${TEMP_CONFIG}"
(or when both publisher and subscriber or only subscriber are run with wal_debug=on)

So I propose a little fix to the test.


Kind regards,
Ian Ilyasov.

Junior Software Developer at Postgres Professional