Don't mix trip with multiprocessing for now

It seems that mixing the two backends in the test suite results in hangs
due to lingering forkservers and resource managers from
`multiprocessing`? Likely we'll need either 2 separate CI runs to work
or someway to be sure that these lingering servers are killed in between
tests.

tests/conftest.py

@@ -38,7 +38,7 @@ def pytest_generate_tests(metafunc):
         methods = get_all_start_methods()
 
         if platform.system() != "Windows":
-            methods += ['trio_run_in_process']
+            methods = ['trio_run_in_process']
 
         if 'fork' in methods:  # fork not available on windows, so check before removing
             # XXX: the fork method is in general incompatible with

tests/test_cancellation.py

@@ -284,16 +284,21 @@ async def spawn_and_error(breadth, depth) -> None:
 
 
 @tractor_test
-async def test_nested_multierrors(loglevel, start_method):
+async def test_nested_multierrors(loglevel):
     """Test that failed actor sets are wrapped in `trio.MultiError`s.
     This test goes only 2 nurseries deep but we should eventually have tests
     for arbitrary n-depth actor trees.
     """
-    # XXX: forkserver can't seem to handle any more then 2 depth
-    # process trees for whatever reason.
-    # Any more process levels then this and we start getting pretty slow anyway
-    depth = 3
-    subactor_breadth = 2
+    # if start_method == 'trio_run_in_process':
+    depth = 2
+    subactor_breadth = 3
+    # else:
+    #     # XXX: multiprocessing can't seem to handle any more then 2 depth
+    #     # process trees for whatever reason.
+    #     # Any more process levels then this and we see bugs that cause
+    #     # hangs and broken pipes all over the place...
+    #     depth = 1
+    #     subactor_breadth = 2
 
     with trio.fail_after(120):
         try: