Why won't sbt run ScalaTest's example test?

Please assume the normal "newbie" appologies.

I'm using scala 2.12.10; In build.sbt I added ScalaTest:

libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.8" % "test"

I'm adding some scala.js to an existing java project so my scala source paths are normal but I made some empty directories for java and resources and use a different target so there would be no collision with existing code:

javaSource in Compile := baseDirectory.value / "src/main/scalajs_java"
javaSource in Test := baseDirectory.value / "src/test/scalajs_java"

resourceDirectory in Compile := baseDirectory.value / "src/main/scalajs_resources"
resourceDirectory in Test := baseDirectory.value / "src/test/scalajs_resources"

target := baseDirectory.value / "scalajs_target"

I put the ScalaTest example file ExampleSpec.scala in src/test/scala/ExampleSpec.scala. The example ran fine using org.scalatest.run as described in http://www.scalatest.org/.

The test classpath looks reasonable, I thought:

sbt:RedsPro-ScalaJS> show test:fullClasspath
[info] * Attributed(/Users/tballard/git/redspro/reds/scalajs_target/scala-2.12/test-classes)
[info] * Attributed(/Users/tballard/git/redspro/reds/scalajs_target/scala-2.12/classes)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.12.10.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scala-js/scalajs-library_2.12/jars/scalajs-library_2.12-0.6.31.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scala-js/scalajs-dom_sjs0.6_2.12/jars/scalajs-dom_sjs0.6_2.12-0.9.7.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scala-js/scalajs-test-bridge_2.12/jars/scalajs-test-bridge_2.12-0.6.31.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scala-js/scalajs-test-interface_2.12/jars/scalajs-test-interface_2.12-0.6.31.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scalatest/scalatest_2.12/bundles/scalatest_2.12-3.0.8.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scalactic/scalactic_2.12/bundles/scalactic_2.12-3.0.8.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scala-lang/scala-reflect/jars/scala-reflect-2.12.10.jar)
[info] * Attributed(/Users/tballard/.ivy2/cache/org.scala-lang.modules/scala-xml_2.12/bundles/scala-xml_2.12-1.2.0.jar)

After compiling, the test is where I would expect it:

> ls scalajs_target/scala-2.12/test-classes/
ExampleSpec.class   ExampleSpec.sjsir   JS_DEPENDENCIES     org/

However, sbt apparently isn't convinced there are any tests:

sbt:RedsPro-ScalaJS>  show definedTestNames
[info] *
[success] Total time: 1 s, completed Dec 1, 2019, 11:37:51 AM

sbt:RedsPro-ScalaJS> testOnly ExampleSpec
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0
[info] No tests to run for Test / testOnly
[success] Total time: 1 s, completed Dec 1, 2019, 12:44:35 PM

A point in the right direction would be greatly appreciated.