build.gradle: upgrade TestFX dependency to 4.0.15-alpha
build.gradle: upgrade TestFX dependency to 4.0.15-alpha

The current version of TestFX dependency we are using is 4.0.12-alpha.

TestFX version 4.0.15-alpha has been announced for a while and it gives
better support for JDK11[1], which we may wish to upgrade to in the
future.

To keep us updated with TestFX's development, let's upgrade TestFX
dependency to its latest stable version 4.0.15-alpha.

[1] https://github.com/TestFX/TestFX/issues/640#issuecomment-436079910

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