Use openci-testing branch

Change-Id: I20c24374b2ba46e707b71eb8ec2e804ea67b0ccb
diff --git a/mbed-tls-nightly-tests.yaml b/mbed-tls-nightly-tests.yaml
index cff215a..b960d31 100644
--- a/mbed-tls-nightly-tests.yaml
+++ b/mbed-tls-nightly-tests.yaml
@@ -49,7 +49,7 @@
       scm:
       - git:
           branches:
-          - main
+          - dev/dgreen-arm/openci-testing
           credentials-id: mbedtls-github-ssh
           url: ssh://git@github.com/Mbed-TLS/mbedtls-test.git
       script-path: vars/mbedtls-release-Jenkinsfile