This test happens to fail if it runs after test
galera_inject_bf_long_wait.
And the reason is that galera_bf_lock_wait greps for message
"BF lock wait long" in the error log, and expects that grep matches
no lines. Whereas galera_inject_bf_long_wait intentionally causes the
message to appear in the log. The fix consists in using
assert_grep.inc with option assert_only_after, such that
galera_bf_lock_wait is limited to grep only those lines that appeared
in the log after it started to execute.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>