Slash-escrow testing

[Bill-Kunj]

The new multi-sig slash-escrow implementation for the RPC custom build (rchain/rchain#3642) requires testing before RPC can launch
https://github.com/rchain/rchain/blob/feature/v0.12-rpc/casper/src/test/resources/MultiSigVault/posMultiSigTransfer.rho
https://github.com/rchain/rchain/blob/feature/v0.12-rpc/casper/src/test/resources/MultiSigVault/posMultiSigConfirm.rho

[Bill-Kunj]

This test requires a transfer into the slash-escrow account before it can be verified.
Code for getting the slash-escrow REV address with exploreDeploy (thank you @zsluedem):

new return, rl(`rho:registry:lookup`),
  poSCh,
  addrCh  
in {
  rl!(`rho:rchain:pos`, *poSCh) |
  for(@(_, PoS) <- poSCh) {
    @PoS!("getCoopVault", *addrCh)|
    for (@(_,addr,_) <- addrCh){
      return!(addr)
    }
 }
}