Make the CI record and track the unit tests

Summary:
This prevents running the tests in parallel but make the CI report
useful info in case of failure and allow for tracking the flaky tests.

Test Plan:
See result on CI.
Running the build-diff configuration should create 2 different junit files, with different test suite names.
In the CI UI, the Tests tab should display each test case individually, associated with its test suite.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: markblundeberg, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5042