Limit windows-specifc test to windows targets only

Test os__process__priority_win32 is a windows test, however its
yaml file used the build os to limit its runs to windows. Change
that to target os in order to account for testing now running on
windows-hosted cross-linux targets.

Issue: gnatcoll-core#6

Author: Aleksandra Pasek

testsuite/tests/os/process/priority_win32/test.yaml

@@ -1,4 +1,4 @@
 title: GNATCOLL.OS.Process Priority Tests (High Level API)
 control:
-    - ["SKIP", "env.build.os.name != 'windows'", "Win32 only test"]
+    - ["SKIP", "'windows' not in env.target.os.name", "Win32 only test"]
 data: ["test_data/*.py"]