When Kotlin tests fail with "class not found" in Android Studio

Today I was setting up a new Android Studio project. For reasons, the app had to be in Java, but I wanted to write the test suite in Kotlin. The test class and test methods had the run buttons in the margin, and the build appeared to go smoothly, but trying to run them produced a "class not found" error instead of test results. It turns out I had forgotten to apply the kotlin-android plugin. (That plugin comes from the usual Kotlin buildscript dependency.) It's also important to add the Kotlin standard library as a normal dependency, as testImplementation in my case.