Create walletdir if datadir doesn't exist and fix tests

Summary:
Wallet dir defaults to <data_dir>/wallets/ if the data directory does
not exist.

Backport 3/4 of core PR11466 (most of commit 8263f6a)

Depends on D2314

Test Plan:

make check

Run the following commands (warning, will rm your local bitcoin data dir
!):

rm -Rf ~/.bitcoin
bitcoind

Check the directory ~/.bitcoin/wallets has been created

./test/functional/test_runner.py keypool-topup multiwallet wallet_hd

walletbackup

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2315