Run scapegoat analysis on test files

[Michael Wizner]

Currently, we limit the scope of the analysis to source files, but we should also analyze test files located in the directories defined by the sonar.tests property (defaults to src/test/scala).

It appears that Scapegoat doesn't allow running against the test scope, so we would need to address scapegoat-scala/sbt-scapegoat#46 first. This might also require parsing a separate scapegoat report file.