Fixes #6515.
The rpc_addmultisig test was failing because cs_wallet was locked prior to rpc functions, upsetting the locking order.
The rpc_wallet test wasn't failing, but I dropped the lock on cs_wallet for consistency.
This reverts commit a4fe57da6207c1e5691a1e843d22db571f3f0186.
The issue here should be fixed by the previous commit.
ACK