Add project level content trust case (#4964)

Add project level content trust testcase

tests/testcases/Group9-Content-trust/9-30-Project-level-content-trust.md

@@ -0,0 +1,40 @@
+Test 9-30 Project enable content trust
+=======
+
+# Purpose:
+
+To verify project level content trust works correctly.
+
+# References:
+User guide
+
+# Environment:
+
+* This test requires one Harbor instance is runnning and available.  
+* A Linux host with Docker CLI installed (Docker client).  
+
+# Test Steps:
+**NOTE:**  
+In below test, <harbor_ip> should be replaced by your harbor's ip or FQDN. If you are using a self-signed certificate,make sure to copy the CA root cert into ```/etc/docker/certs.d/<harbor_ip>``` and ```$HOME/.docker/tls/<harbor_ip>:4443/```  
+project a should be replaced by meaingful and longer name.  
+
+1. Login UI and create a project a.  
+2. Push an image to project a.  
+3. On Docker clinet, run  
+```sh
+export DOCKER_CONTENT_TRUST=1
+export DOCKER_CONTNET_TRUST_SERVER=https://<harbor_ip>:4443
+```
+and login Harbor.  
+4. Push an image to project a.  
+5. In project a configuration page, enabled project level content trust.  
+6. Pull the image the first time pushed.   
+7. Pull the image the second time pushed.  
+
+# Expected Outcome:
+
+* In step6, the image can not be pulled.  
+* In step7, the image can be pulled. 
+
+# Possible Problems:
+None