The test fails on random secret with backslash.
The random handlersocket_plain_secret is generated from the '0'..'z'
range, which includes the backslash. The test masked the secret with
--replace_result $HS_SECRET, and mysqltest parses --replace_result
arguments with backslash escapes, so the search string lost the
backslash and did not match the output. The unmasked secret then
appeared in the result (about 19% of runs).
The fix masks the secret in SQL: IF(variable_name LIKE
'handlersocket_plain_secret%', 'HS_SECRET', variable_value), which does
not depend on the secret value.
30e7c940ff