Merge #11210: Stop test_bitcoin-qt touching ~/.bitcoin

Summary:
dea086f49 Stop test_bitcoin-qt touching ~/.bitcoin (MeshCollider)

Pull request description:

Fixes https://github.com/bitcoin/bitcoin/issues/11192

The directory remains unused, but this stops the tests touching ~/.bitcoin at all (namely creating it if it doesn't exist)

Tree-SHA512: e59ad6b83dbc5ea2fb2761994c09933721d29668b0eef09b9d938a4ee1c67871c5125c57483ee0ea25f2385e308d275d86bcb9087dd4d502923013b4f3dbac82

Backport of Core PR11210

https://github.com/bitcoin/bitcoin/pull/11210/files

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3442