gitlab-ci.yml: Test the dtrace backend in one of the jobs

We are using the dtrace backend in downstream RHEL, so testing this
in the CentOS 8 task seems to be a good fit.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210331160351.3071279-1-thuth@redhat.com>
Message-Id: <20210401102530.12030-12-alex.bennee@linaro.org>