Add core/test/README.md
Add core/test/README.md

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