Update unit tests to use python3

Various CB Server use python for test harnesses and similar. Some of these assume /usr/bin/python is actually Python 2.x.

As of Ubuntu20.04 (and possibly other distros), /usr/bin/python is Python3, and hence if a given program is not written to work with either python 2 or python 3, it will fail when run on Ubuntu 20.04.

For example, the following unit test failures are pretty much all due to this issue: http://cv.jenkins.couchbase.com/job/unit-simple-test/32081/testReport/

We need to update the affected test programs; ideally to migrate them to work with python3 and explicitly request python3 as their interpreter.