scripts: twister: Fix Unit Tests on Windows systems

Unit tests currently are not runnable on Windows systems, failing
on two testfiles: test_jobserver.py and test_testsuite.py.

This commit removes code dependency on Windows-unavailable
elements on Windows systems via skipping the offending tests.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>
2 files changed