add explicit JUnit dependency

this does two things:
* gets us on latest JUnit instead of an old one
* allows removing a weird workaround from this repo's entry
  in the Scala community build (though I don't understand why)

Author: SethTisue

build.sbt

@@ -22,6 +22,7 @@ lazy val junit = project.in(file("junit"))
   .settings(commonSettings)
   .settings(
     libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % Test,
+    libraryDependencies += "junit" % "junit" % "4.13.1" % Test,
     // for javax.xml.bind.DatatypeConverter, used in SerializationStabilityTest
     libraryDependencies += "javax.xml.bind" % "jaxb-api" % "2.3.1" % Test,
     testOptions += Tests.Argument(TestFrameworks.JUnit, "-a", "-v"),