Add a test for reproducible/env independent builds

[romainmenke]

Not all bundlers create reproducible/env independent builds.

Creating a build from the same source code with the same dependencies should generate the same output.
Even if the path to the source code changes or the environment in which the build runs is changed.

• my machine vs your machine
• my machine vs build server
• rebuild after time passes

[surma]

We do have a test to check that only the relevant bits of the path to source are considered.

I like the idea of having another test in the same category that tests if the same hashes are generated across different operating systems (Linux, Linux in Docker, Windows, Mac).