[Storage] SchemaDB: add tests for open_readonly
[Storage] SchemaDB: add tests for open_readonly

Closes: #3365
Approved by: msmouse

aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core