Improve unit tests

[chrhansk]

There are as of currently some inconveniences with regards to the built-in unit tests:

• First, the tests are installed, which goes against conventions: Either people help to develop the package using the existing source, or they want to use the library / drivers after install. In either case, the tests are not required to be installed.
• Second, the unit tests don't pass unless certain OMP-related environment variables are set. If these variables are set via meson, then this problem disappears and the tests always pass.

[amontoison]

I was not aware that it was possible to pass environment variables to Meson like this. Thanks for the tip, @chrhansk!