Consistency fixes for test_executor_sequential (#566)

A few things here:
1. Use a real Job object like the other test. The sequential executor
   doesn't seem to need it to have a 'identifier' property, but other
   executors do and we should be consistent.
2. Move stuff out of the try/finally block for the os.kill thread to
   reduce the likelihood of secondary exceptions.
3. Clear the global job accumulation variable even though we don't use
   check it (which is another conversation entirely).

test/test_executor_sequential.py

@@ -148,10 +148,17 @@ def test_sequential():
     ran_jobs.clear()
 
 
-async def job8():
-    global ran_jobs
-    await asyncio.sleep(1)
-    ran_jobs.append('job8')
+class Job8(Job):
+
+    def __init__(self):
+        super().__init__(
+            identifier='job8', dependencies=set(), task=None,
+            task_context=None)
+
+    async def __call__(self, *args, **kwargs):
+        global ran_jobs
+        await asyncio.sleep(3)
+        ran_jobs.append(self.identifier)
 
 
 @pytest.fixture
@@ -174,21 +181,24 @@ def test_sequential_keyboard_interrupt(restore_sigint_handler):
     args = None
     jobs = OrderedDict()
     jobs['one'] = Job1()
-    jobs['aborted'] = job8
+    jobs['aborted'] = Job8()
     jobs['four'] = Job4()
 
     def delayed_sigint():
         time.sleep(0.1)
+        # Note: a real Ctrl-C would signal the whole process group
         os.kill(
             os.getpid(),
             signal.SIGINT if sys.platform != 'win32' else signal.CTRL_C_EVENT)
         if sys.platform == 'win32':
             os.kill(os.getpid(), signal.CTRL_C_EVENT)
 
     thread = Thread(target=delayed_sigint)
+    thread.start()
     try:
-        thread.start()
         rc = extension.execute(args, jobs)
-        assert rc == signal.SIGINT
     finally:
         thread.join()
+
+    assert rc == signal.SIGINT
+    ran_jobs.clear()