Use build directory for functional test junit results

Summary: Use build_dir for test output directory.

Test Plan: Run functional tests and see where junit_results.xml is placed.

Reviewers: matra774, matiu, #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Maniphest Tasks: T243

Differential Revision: https://reviews.bitcoinabc.org/D1118