Add tests to cover mixed predicate use

The new test cases cover:
 - `when` passes immediately and `until` is called five times
 - `when` is called five times and `until` passes immediately
 - `when` passes immediately and `until` fails with exponential interval
 - `when` fails with exponential interval and `until` passes immediately
Merge pull request #5 from adlawson/test-mixedpredicate

Add tests to cover mixed predicate use