[Test] Fix multiwallet test with Boost 1.78+
[Test] Fix multiwallet test with Boost 1.78+

`fs::create_directories` now has an error message saying
`create_directories` instead of `create_directory`. This caused
wallet_multiwallet.py to fail. The error message check has been updated
to be able accept either string.

PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX