MDEV-33768: Memory leak found in the test main.constraints run with --ps-protocol against a server built with the option -DWITH_PROTECT_STATEMENT_MEMROOT

The discovered memory leak was introduced by the commit
  762bf7a0
    (MDEV-22602 Disable UPDATE CASCADE for SQL constraints)

The reason why a memory leaked on running the test main.constraints
is that a statement arena was used for allocation a memory
for storing a constraint name. A constraint name is an entity having
temporary nature by its design so runtime arena should be used for its
allocation.