Gradle support for getting :web:test working properly

[Jesper Skov]

Hi

I struggled somewhat to get :web:test running with -PSTUB_RUNTIME.

The JVM kept crashing by what turned out to be missing media
libraries (the failure message was hidden).

I tried building with -PCOMPILE_WEBKIT=true, but it takes a terrible
long time on my laptop. And did not in itself fix the problem.

Frustrations and lost time was the only real outcome of this :)




So I would suggest adding logic to the build file to allow something
like:

    gradlew -PSTUB_RUNTIME_USE=15-ea+4 all test

This should download org.openjfx:javafx-web and
org.openjfx:javafx-media artifacts in the specified version.

Then unpack the shared libraries to a build folder, and make them
availble via the STUB_RUNTIME logic.


Plus an addition to the CONTRIBUTING.md documenting this.


I would be happy to help make and/or test the changes, but am only
able to work on Linux.


Thanks,
Jesper