message: [core] Add set_caller to test env api
diff: //! Public api to interact with the special testing environment.
  use super::ContractEnv;
  +use crate::env::Address;
  /// Returns the total number of reads to all storage entries.
  pub fn total_reads() -> u64 {
  @@ -27,3 +28,8 @@ pub fn total_reads() -> u64 {
  pub fn total_writes() -> u64 {
  ContractEnv::total_writes()
  }
  +
  +/// Sets the caller for the next calls to the given address.
  +pub fn set_caller(address: Address) {
  + ContractEnv::set_caller(address)
  +}
  