fixup

I tried to get `UsingTest` passing on Scala 3. on the JVM, this was
sufficient to get the 2.13.17 stdlib so that the tests pass:
  dependencyOverrides += "org.scala-lang" % "scala-library" % scala213
but it doesn't do the trick on JS or Native for reasons unknown.
so I worked around by putting `UsingTest` under `src-jvm`.
once we are on a Scala 3.3 LTS version that has taken the 2.13.17
upgrade, it could be moved to regular `src`. (but anyway, who cares...)

Author: SethTisue