Combine test-downstream and upgrade-downstream features

[mockitoguy]

Problem

Currently the downstream projects that we test against is a separate list than project that we perform downstream upgrade. It feels that there should be single configuration for this.

Additionally, we are cloning the same repositories multiple times because they are not reused between downstream testing and downstream upgrading.

Brainstorming

This is just brainstorming, the idea is to combine the features cleanly, without coupling them too much. It feels that when we have completed the downstream test, it is a good candidate for downstream upgrade.

apply plugin: 'org.shipkit.test-downstream'

testDownstream.testOnly("mockito/mockito") //test only
testDownstream.upgrade("mockito/shipkit", "mockito/shipkit-example") //test & upgrade

[mockitoguy]

@wwilk, thoughts?

[wwilk]

@szczepiq Yeah, it certainly needs to be done. I will work on it soon.

[wwilk]

Important to consider #438 when working on this one