Travis: Build pipeline to build once, test multiple times?

All of our builds first create and build the app before actually testing it:

As these are all the same (per platform), it would make sense and stuff much faster if we could build it all once, then use the built app in the different environments to run the tests on each platform individually.

Start by reviewing the Travis build configuration and the platform-specific build and test steps shown in the issue. Trace where each platform builds the app, then verify that the app is built once per platform and reused across the corresponding test environments.

Written by the indexing model from the issue text.