Make it easier to run tests for installed package

[V02460]

Is your feature request related to a problem? Please describe.
I’m currently packaging this library for Fedora. Building an OS package includes running its tests on the system-installed modules. By having the matrix_common and tests modules in the same folder, the local module will shadow the system-installed one when running the tests. This can lead to false-positive test results.

Describe the solution you'd like
Consider moving matrix_common into a src folder. This way the tests module can be added to the Python path independently of anything else, preventing shadowing when running the tests.

Describe alternatives you've considered
Maintain a downstream workaround by moving/removing files before testing.