Revert change to test skip condition

This was part of a commit cherry picked from another branch, and this
change didn't make sense on this branch.

IPython/kernel/tests/test_kernelspec.py

@@ -56,9 +56,7 b' class KernelSpecTests(unittest.TestCase):'
                                      kernel_name='tstinstalled',
                                      replace=True, user=True)
 
-    @onlyif(kernelspec.find_install_prefix() in {'/usr', '/usr/local'}
-            and not os.access('/usr/local/share', os.W_OK),
-            "needs Unix system without root privileges")
+    @onlyif(os.name != 'nt' and not os.access('/usr/local/share', os.W_OK), "needs Unix system without root privileges")
     def test_cant_install_kernel_spec(self):
         with self.assertRaises(OSError):
             self.ksm.install_kernel_spec(self.installable_kernel,