Refactor multi-JVM testing

[sgammon]

Jake Wharton has a great blog post about testing on different JVM versions from Gradle rather than using a build matrix. We should do that!

[sgammon]

this got a bit more complex: we'll need copies of GraalVM, but there is no download plugin yet, so we will probably need to write our own. furthermore, there does not seem to be a way to "prepare" the JVM other than installing it to disk (i.e. running gu install * may be challenging).