Allow using different Erlang versions for test-build and running tests

[essen]

The use case is for example RabbitMQ which distributes beam files compiled with (for example) 19.3 that must work on 20.3 and 21.3 as well. So the beam file must be working on all versions moving forward.

Erlang.mk should provide a mechanism to easily perform that kind of test run (similar to ci except the test build is not removed between builds).