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>
Would be good to add more parentheses here, so clearer that first three conditions are a group.

Personally, I'd probably go for the code below. It duplicates the exception line, but I think the if logic is simpler to read which compensates. No particularly strong feelings though:

        if (isWinPlatform && isUnixTest && !isWinTest) {
            throw new SkipException("Skipping unix-specific test");
        } else if (!isWinPlatform && isWinTest && !isUnixTest) {
            throw new SkipException("Skipping windows-specific test");
        }