[storage] rename should_exist to should_exist_vec in schemadb test
[storage] rename should_exist to should_exist_vec in schemadb test

Closes: #2498
Approved by: davidiw

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