[CI] Prevent copying undesired test/tmp content to the artifacts

Summary:
All we want is the test_runner_* content in case of a functional test
failure.

Test Plan:
Run any CI build with both functional and unit tests. Edit a functional
test so it fails and check the artifacts only contains that failing
test.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7076