Merge #14146: wallet: Remove trailing separators from -walletdir arg

Summary:
2d471636eb9160ab51b08e491e3f003f57adbc36 wallet: Remove trailing separators from -walletdir arg (Pierre Rochard)
ea3009ee942188750480ca6cc273b2b91cf77ded wallet: Add walletdir arg unit tests (Pierre Rochard)

Pull request description:

If a user passes in a path with a trailing separator as the `walletdir`, multiple BerkeleyEnvironments may be created in the same directory which can lead to data corruption.

Discovered while reviewing https://github.com/bitcoin/bitcoin/pull/12493#issuecomment-417147646

Tree-SHA512: f2bbf1749d904fd3f326b88f2ead58c8386034355910906d7faea155d518642e9cd4ceb3cae272f2d9d8feb61f126523e1c97502799d24e4315bb53e49fd7c09

Backport of Core PR14146
https://github.com/bitcoin/bitcoin/pull/14146/files

Test Plan:
ninja check
Autotools build on CI

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5004