ceph-build-pull-requests: pass --config-xml to "jenkins-jobs test"

ceph-build-pull-requests: pass --config-xml to "jenkins-jobs test"

to silence warnings like

WARNING:jenkins_jobs.cli.subcommand.test:(Deprecated) The default output
behavior of `jenkins-jobs test` when given the --output flag will change
in JJB 3.0. Instead of writing jobs to OUTPUT/jobname; they will be
written to OUTPUT/jobname/config.xml. The new behavior can be enabled by
the passing `--config-xml` parameter.

also redirect the test result to /dev/null, otherwise the test will fail
because the output directory may already exist.

Signed-off-by: Kefu Chai <kchai@redhat.com>