Add framework testing multibranch job

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: If68ec2208199e36779034d44757ea4970338fe14
diff --git a/mbed-tls-framework-restricted-multibranch.yaml b/mbed-tls-framework-restricted-multibranch.yaml
index 594a54f..1c07306 100644
--- a/mbed-tls-framework-restricted-multibranch.yaml
+++ b/mbed-tls-framework-restricted-multibranch.yaml
@@ -44,7 +44,7 @@
         discover-pr-forks-strategy: both
         discover-pr-forks-trust: nobody
         discover-pr-origin: both
-        head-filter-regex: 'PR-\d+-head|gh-readonly-queue/.*'
+        head-filter-regex: '(?!^PR-1-(head|merge)$)PR-\d+-head|gh-readonly-queue/.*'
         status-checks:
           skip: true
           skip-branch-source-notifications: true