Restrict framework-multibranch to test PR

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: I9ef022b261ddae172c8a29d2fac7949d12ca1107
diff --git a/mbed-tls-framework-multibranch.yaml b/mbed-tls-framework-multibranch.yaml
index 68d18c5..0ae68b2 100644
--- a/mbed-tls-framework-multibranch.yaml
+++ b/mbed-tls-framework-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-33-head|gh-readonly-queue/.*'
         status-checks:
           skip: true
           skip-branch-source-notifications: true