Make PBS tests more tolerant for SIGTERM variants

At least on RHEL8 and Python 3.11 the return code from PBS has been observed to
be 128 + SIGTERM while at least on RHEL7/Python 3.8 it is 256 + SIGTERM.

We should accept both variants in tests.

tests/integration_tests/scheduler/test_generic_driver.py

@@ -79,20 +79,20 @@ async def finished(iens, returncode, aborted):
 async def test_kill(driver):
     aborted_called = False
 
-    expected_returncode = 1
+    expected_returncodes = [1]
     if isinstance(driver, OpenPBSDriver):
-        expected_returncode = 256 + signal.SIGTERM
+        expected_returncodes = [128 + signal.SIGTERM, 256 + signal.SIGTERM]
 
     if isinstance(driver, LocalDriver):
-        expected_returncode = -signal.SIGTERM
+        expected_returncodes = [-signal.SIGTERM]
 
     async def started(iens):
         nonlocal driver
         await driver.kill(iens)
 
     async def finished(iens, returncode, aborted):
         assert iens == 0
-        assert returncode == expected_returncode
+        assert returncode in expected_returncodes
         assert aborted is True
 
         nonlocal aborted_called