Fix State Trie Migration Benchmarks (#11502)

* enable signed migrations in benchmarks

* T instead of Test

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Remove 'mut'

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>