Increase randomness of course name in testcase

@benpatterson or @clytwynec pls review.
Grepping through the code, it seems that a lot of our tests use 5 hex digits, so I increased it to be the same.

@nasthagiri FYI. This test [failed intermittently](https://ci.solanolabs.com/reports/963416), it seems the root cause was that 3 hex digits wasn't of sufficient random uniqueness.