Kindly follow the below link for workaround

https://success.salesforce.com/issues_view?id=a1p30000000T0kUAAS

Note:
1. Kindly check whether @isTest notation is added.
2. Go to Setup –> Apex Test Execution –> View Test History and click “Clear Test Results” button.

After following the above steps, if the problem persists,

Kindly use following troubleshooting steps:

1. Disable Parallel Apex Testing
2. Clear apex test execution history 
3. Compile all classes

Cheers!!!