MDEV-32655: rpl_semi_sync_slave_compressed_protocol.test assert_only_after is wrong

The MTR test rpl.rpl_semi_sync_slave_compressed_protocol scans the
log file to ensure there is no magic number error. It attempts to
only scan the log files of the current test; however, the variable
which controls this, , is initialized incorrectly,
and it thereby scans the entire log file, which includes output from
prior tests. This causes it to fail if a test which expects this
error runs previously on the same worker.

This patch fixes the assert_only_after so the test only scans
through its own log contents.

mysql-test/suite/rpl/t/rpl_semi_sync_slave_compressed_protocol.test

@@ -40,7 +40,7 @@ DROP TABLE t1;
 --let $assert_select=Read semi-sync reply magic number error
 --let $assert_file= $MYSQLTEST_VARDIR/log/mysqld.1.err
 --let $assert_count= 0
---let $assert_only_after = CURRENT_TEST:rpl.rpl_semi_sync_slave_compressed_protocol.test
+--let $assert_only_after = CURRENT_TEST: rpl.rpl_semi_sync_slave_compressed_protocol
 --source include/assert_grep.inc
 
 --connection master