Arthur: update pr head job for testing

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I6f37e52fb606db6c010fbf4a1bbea820d86d4fbe
diff --git a/arthur-mbed-tls-pr-head.yaml b/arthur-mbed-tls-pr-head.yaml
index 8bf970a..5a9b7f6 100644
--- a/arthur-mbed-tls-pr-head.yaml
+++ b/arthur-mbed-tls-pr-head.yaml
@@ -3,7 +3,7 @@
     description: 'Job for testing mbed tls PR heads'
     project-type: multibranch
     prune-dead-branches: true
-    days-to-keep: 60
+    days-to-keep: 70
     num-to-keep: 1000
     script-path: tests/.jenkins/Jenkinsfile
     properties: