Add running tests to install-guide

At least on architectures we don't test, it is necessary that the user
runs the tests. Because we don't have a way to query whether it is a
tested architecture every user should run the tests.

Also add a warning if unit tests cannot be build because libmxl2 wasn't
found.

Partial fix for #1548

Change-Id: I8b87cc77f909d5cf2a4ee70151096312e3191af2