fix linux CI

[UnGast]

Description

CI fails on linux because Platform.initialize() is called in the testPlatformSetup() test case, which seems to require a video device.

possible fixes:

• don't execute tests on linux (current solution)
• add environment variable, read it in Package.swift and add a define to specifically exclude tests that require a video device
• setup a virtual video device (quite a lot of configuration work probably, but doable (have already done it at some point))

Which solution to pick depends on how important the tests are to this project. Is there a real need to write detailed tests as I assume that SDL2 is already being tested?
If the answer is that there is a need the CI could run without tests until there is a significant amount of them.

Checklist

• I've read the Contribution Guidelines
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (to the extent possible).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexpected benchmark regressions.
• I've updated the documentation (if appropriate).

[ctreffs]

Let's keep the solution for this as is for now.

setup a virtual video device (quite a lot of configuration work probably, but doable (have already done it at some point))
will be done in the future

[ctreffs]

Will close: #2