10-08  user can not pull image exceed vulnerability severity
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# Purpose:  
To verify user can not pull image exceed vulnerability severity setting. 

# Reference:
User guide.  

# Environment:
* This test requires that one Harbor instance is running and available.  
* Harbor is installed with trivy enable.  
* A Linux host with Docker client installed.  
* Trivy has been updated to the latest.  

# Test Step:
1. Login harbor as admin.  
2. Go to configuration.  
3. Set vulnerability severity limit to medium and save configuration.  
4. Push some images with vulnerability and scan them.  
5. On a docker client, user pull an image with high vulnerability severity. 

# Expect outcome:
* Step5 pull request should be refused.  

# Possible problem:
None
