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.

This change is required to have scala/scala#2859 passing the Scala/Scala IDE pr validation.