Fix log_statement_max_length test with verbose logs

Fix log_statement_max_length test with verbose logs

Buildfarm member prion reported a failure in the test added by commit
c8bd8387c27 to verify that the server logs an empty statement body
when log_statement_max_length = 0.

The test assumed that "statement:" would appear immediately after
"LOG:" in the logged statement message. However, prion runs with
log_error_verbosity = verbose, which inserts the SQLSTATE between
"LOG:" and the message text. As a result, the test failed even though
the server behaved correctly.

Per buildfarm member prion.

Discussion: https://postgr.es/m/CAHGQGwFiQKwvLVG+U0WWNo2kgkQ88FVGhYH_MBZu9Y0SJ8BjDw@mail.gmail.com