[CMAKE] Properly clean the junit reports

Summary:
The merged test suite junit reports from test/junit are correclty
cleaned, but not the intermediate test reports located in the current
binary directory.

Test Plan:

ninja check

Check there is a lot of xml reports here:

ls src/test

Then:

ninja clean
ls src/test

Check the reports are gone as expected.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7223