These tests use search_pattern_in_file.inc to search the error log for
expected output. However, search_pattern_in_file.inc by default searched only
the first 50000 bytes, so if the error log grew too big the tests would fail.

This patch extends search_pattern_in_file.inc with an option to specify how
much of the file to search, and whether to search from the start of the file
or from the end. Then the rpl.rpl_checksum and rpl.rpl_gtid_errorlog test
cases are fixed to search the last 50000 bytes of the error log, which will
work no matter how large prior tests have made it.