jdk9 tests do not run

[pjfanning]

See https://github.com/apache/pekko/pull/1432 - I deliberately modified 2 classes (1 Java, 1 Scala) to see if they were even compiled. The PR did fail because scalafmt and the doc build failed but the main part of the build where we compile code and run tests - this all ran ok.

[Roiocam]

I guess sbt dependWalkerCheck didn't want to verify TestJDK9+ classes compile, but I don't remember why it was on purpose.

and it is normal that PRValidationTest does not run JDK9 verification. We will not verify these on the main process, but only verify on binary compability or documentation check

[pjfanning]

#1434 merged