Use new sbt syntax for defining `check-compilations` task.

Some scripted tests define `check-compilations` task. We can refactor them
to be more readable by using new `.value` syntax.
Update comments in `inc-ant-style` scripted test.

I forgot to update comments after copy&pasting the test. The updated
comments describe what's being tested related to Ant mode.
Merge pull request #1328 from gkossakowski/cleanup-incremental-tests

Cleanup incremental tests