Refactor some magic constants in PreferencesTest.
Refactor some magic constants in PreferencesTest.

LedgerHQ/lib-ledger-core,LedgerHQ/lib-ledger-core,LedgerHQ/lib-ledger-core,LedgerHQ/lib-ledger-core,LedgerHQ/lib-ledger-core,LedgerHQ/lib-ledger-core,LedgerHQ/lib-ledger-core