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, $assert_only_after, is initialized incorrect, 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.