SI-7825 Consider DEFAULTMETHOD when refchecking concreteness

A class should not be required to implement a Java default method.

This commit uses `isDeferredNotDefault` in place of `isDeferred`
when finding unimplemented methods.

The test itself does not depend on Java 8 as we use scalac's
Java source parser to set things up.
SI-7398 Enable test for Java 8 source parser under Java 8

There is no need to skip it as it only depends on our changes
to our JavaParser, and not on any bytecode features of Java 8.