add a scala_check target that only uses scalacheck.

This could be useful for people that already have scalacheck tests and don't want to convert to scalatest to use them. Also, the dependencies of scalacheck are very thin.

The issue names a new scala_check target and existing ScalaCheck and ScalaTest tests, but no files or test entry points. Start by locating the repository's existing ScalaTest Bazel target and its dependency definitions, then determine how a ScalaCheck-only target should differ. Done means users can run ScalaCheck tests through the new target without requiring ScalaTest.

Written by the indexing model from the issue text.