rule's "scalacopts" should get higher priority than toolchain "scalacopts"

[or-shachar]

In scalacopts there are some options that can be overridden by their last occurrance and some options that are aggregated

We need a test that makes sure that options that come from library rule scalacopts comes after any option that comes from toolchain's scalacopts

On #364 I did not add such test and I think it should be added.