Fix walletdb_tests on windows

Summary:
Windows is a bit more restrictive when it comes to files not properly closed and alike. This caused an error in the cleanup after running hte test (the DB files were kept open and couldn't be deleted).

This ensure that the DB is closed properly after use. The use of MockDB ensure that files shared accross DB instance such as db.log do get mamaged in memory instead of on disk.

Test Plan:
Cross compile windows binaries, then

wine src/test/test_bitcoin.exe -t walletdb_tests

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D791