Merge #13667: wallet: Fix backupwallet for multiwallets

Summary:
a1a998cf24c0cf1232e44ec8eaf2ad6875ab5153 wallet: Fix backupwallet for multiwallets (Daniel Kraft)

Pull request description:

`backupwallet` was broken for multiwallets in their own directories (i.e. something like `DATADIR/wallets/mywallet/wallet.dat`).  In this case, the backup would use `DATADIR/wallets/wallet.dat` as source file and not take the specific wallet's directory into account.

This led to either an error during the backup (if the wrong source file was not present) or would silently back up the wrong wallet; especially the latter behaviour can be quite bad for users.

Tree-SHA512: 7efe2450ca047e40719fcc7cc211ed94699056020ac737cada7b59e8240298675960570c45079add424d0aab520437d5050d956acd695a9c2452dd4317b4d2c4

Backport of Core PR13667
https://github.com/bitcoin/bitcoin/pull/13667/

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4723