The samples "basic" and "advanced" that were migrated from Gradle core contain test code that's incompatible with Play 2.6.15. My guess is that they were built based on an old version of Play. Some of the APIs used in the test classes were removed/replaced completely e.g. FakeRequest. Furthermore, some dependencies are missing. At the very least the following needs to be added:

dependencies {
    playTest 'org.specs2:specs2-tests_2.12:4.3.5-78abffa2e-20181150936'
    playTest 'org.specs2:specs2-junit_2.12:4.3.5-78abffa2e-20181150936'
}

I think we'd need to make significant changes to that sample test code to make it compatible with Play 2.6.