Document testing procedures for unit/functional tests

It would be good to have clear documentation in the repo about how to:

(1) run individual tests
(2) change cluster parameters
(3) write more tests

This might be trivial or obvious to do, but very much needed.

For an example of what should be available, see:

(This does not say how to run individual tests though)
https://github.com/couchbase/libcouchbase#run-the-testsuite-towards-a-running-cluster

A more extensive example for a more complex test suite:
https://github.com/couchbase/php-ext-couchbase/blob/master/TESTING.pod

I'm marking this as a 'library' bug because this would require some detailed knowledge of how the tests are written