Updates continuations option tests for 2.11

In Scala 2.11, the continuation plugin is no more enabled by default when
available.
Two main changes:
- forces enable the continuation plugin when running the tests on Scala 2.11
- modifies the tests to use the project specific preferences, to avoid side effects
of the configuration changes needed for the tests.
Merge pull request #531 from skyluc/issue/continuations-not-enabled-on-211

Updates continuations option tests for 2.11