Allow jobs to proceed if dependencies are fulfilled

Description

I have a Xamarin Forms app which generates a build for iOS and Android. It also runs the tests. So I have 2 stages (build and test); and 2 jobs in each stage (android and iOS). At this moment the test stage only starts when both android and iOS have completed the build step, which holds up the android tests quite a bit as the iOS build is taking way longer.

Proposal

Start jobs in a next stage when all dependencies in the previous phase have completed.

Overview

This is explained in the description

Use cases

This is explained in the description

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.