The test task is configured to use the runtime java version, but there
are issues with the version of groovy used by gradle pre 6.0. In order
to workaround this, we use the Gradle JDK to execute the build-tools
tests.
Closes#49404Closes#49253
This PR adds build configuration to use the `jdk-download` plugin with
unit tests when no runtime java is configured externally.
It's a first part in a longer chain of changes described in #40531.