gitlab-ci.yml: Merge the trace-backend testing into other jobs

Our gitlab-ci got quite slow in the past weeks, due to the immense amount
of jobs that we have, so we should try to reduce the number of jobs.
There is no real good reason for having separate jobs just to test the
trace backends, we can do this just fine in other jobs, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210319095726.45965-1-thuth@redhat.com>
Message-Id: <20210323165308.15244-11-alex.bennee@linaro.org>