Add tests which demonstrate issue #4

Control-C and/or signalling with SIGINT doesn't result in clean
cancellation and teardown of a program that has spawned subproceses.
These tests demonstrate this issue as is documented in
#4.

tests/core/test_open_in_process.py

@@ -1,5 +1,6 @@
 import pickle
 import signal
+import os
 
 import pytest
 import trio
@@ -14,12 +15,26 @@ async def do_sleep_forever():
 
         await trio.sleep_forever()
 
-    with trio.fail_after(2):
-        async with open_in_process(do_sleep_forever) as proc:
-            proc.terminate()
+    async with open_in_process(do_sleep_forever) as proc:
+        proc.terminate()
     assert proc.returncode == 15
 
 
+@pytest.mark.trio
+async def test_open_in_proc_cancel_via_fail_after():
+    """Test that a ``trio`` initated timeout cancellation fails as expected.
+    """
+    async def do_sleep_forever():
+        import trio
+
+        await trio.sleep_forever()
+
+    with pytest.raises(trio.TooSlowError):
+        with trio.fail_after(0.5):
+            async with open_in_process(do_sleep_forever):
+                pass
+
+
 @pytest.mark.trio
 async def test_open_in_proc_kill_while_running():
     async def do_sleep_forever():
@@ -47,6 +62,42 @@ async def monitor_for_interrupt():
         assert proc.returncode == 2
 
 
+@pytest.mark.trio
+async def test_open_in_proc_cancel_via_SIGINT():
+    pid = os.getpid()
+
+    async def do_sleep_forever():
+        import trio
+
+        await trio.sleep_forever()
+
+    with trio.fail_after(2):
+        async with open_in_process(do_sleep_forever):
+            os.kill(pid, signal.SIGINT)
+            await trio.sleep_forever()
+
+
+@pytest.mark.trio
+async def test_open_in_proc_cancel_via_SIGINT_other_task():
+    pid = os.getpid()
+
+    async def do_sleep_forever():
+        import trio
+
+        await trio.sleep_forever()
+
+    async def spawn_and_sleep_forever(task_status=trio.TASK_STATUS_IGNORED):
+        async with open_in_process(do_sleep_forever):
+            task_status.started()
+            await trio.sleep_forever()
+
+    with trio.fail_after(2):
+        # should never timeout since SIGINT should cancel the current program
+        async with trio.open_nursery() as n:
+            await n.start(spawn_and_sleep_forever)
+            os.kill(pid, signal.SIGINT)
+
+
 @pytest.mark.trio
 async def test_open_proc_invalid_function_call():
     async def takes_no_args():