mysql-util: extend probe tests and make lower bounds mandatory

Tests default to utf8mb4_bin, with utf8mb4_general_ci for the
case-insensitive cases, matching the collations snapshot splitting
accepts. New tests pin the edge cases that rule out other collations:
the Czech ch contraction silently drops prefixes, and NUL keys and
eszett expansions return prefixes that sort below the range they came
from.

Makes the probe lower bounds mandatory, the empty string meaning the
start of the key space, so the root follows the same exclusive-bound
rule as every other walk position.

Author: peterdukelarsen