Skip UNIX-specific tests so they don't break Windows builds.

Ignore platform specific tests. Decide at runtime whether the test should be executed based on the current platform.

<del>
This PR is meant for discussion rather than merging straight away.

Some of the existing tests are clearly UNIX specific. I have created a new test group for them and excluded it explicitly from the tests.
Should I
  * move the tests to the Integration group
  * Find a way to decide at runtime whether the test should be executed based on the current platform
  * or make them pass on windows as well
</del>
I like that the message becomes more informative, implemented.
Thanks for taking the time to review it, these are the last changes preventing me from running the full test suite.