fix multiwallet / symlink check

Summary: fix multiwallet / symlink check when using --tmpdirprefix=

Test Plan: ./test/functional/test_runner.py --tmpdirprefix=output multiwallet

Reviewers: schancel, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1052