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[tests] Change tempdir prefix to something within build environment

Description

[tests] Change tempdir prefix to something within build environment

Summary:
Change tmpdir prefix so that the build system can grab it as an
artifact.

Test Plan: Run locally

Reviewers: deadalnix, matiu, #bitcoin_abc

Reviewed By: matiu, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1034