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

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
I thought that `echo hello world` would work on windows as well - given this is a `ProcessTool` that uses `java.lang.ProcessTool` under the covers. Where does it fail?