Make JSExecutionContexts extend ExecutionContextExecutor
Add original ExecutionContext from Scala lib for tracking

Originates from
scala/scala@a6e77f721921d9e2569433761a1e6a4056ae10f8
Fix #1306: Set JS queue as global ExecutionContext
Merge pull request #1313 from gzm0/global-exec-context

Global exec context
Make RhinoJSEnv final
Centralize overriding of stop in ComJSEnv
Give multiple closed states to RhinoJSEnv.Channel
Fix #1314: Make sure Rhino always closes Com
Improve information in thrown ComClosedExceptions
Fix #1073: New test framework interface (aligned with sbt)
Merge pull request #1289 from gzm0/test-interface

New test interface