Fix cluster suite, chng to new `gather_contexts()`

Namely `test_empty_mngrs_input_raises()` was failing due to
lazy-iterator use as input to `mngrs` which i guess i added support for
a while back (by it doing a `list(mngrs)` internally)? So just change it
to `gather_contexts(mngrs=())` and also tweak the `trio.fail_after(3)`
since it appears that the prior 1sec was causing
too-fast-of-a-cancellation (before the cluster fully spawned) and thus
the expected `ValueError` never to show..

Also, mask the `tractor.trionics.collapse_eg()` usage (again?) in
`open_actor_cluster()` since it seems unnecessary.

tests/test_clustering.py

@@ -13,27 +13,22 @@ MESSAGE = 'tractoring at full speed'
 def test_empty_mngrs_input_raises() -> None:
 
     async def main():
-        with trio.fail_after(1):
+        with trio.fail_after(3):
             async with (
                 open_actor_cluster(
                     modules=[__name__],
 
                     # NOTE: ensure we can passthrough runtime opts
-                    loglevel='info',
-                    # debug_mode=True,
+                    loglevel='cancel',
+                    debug_mode=False,
 
                 ) as portals,
 
-                gather_contexts(
-                    mngrs=(
-                        p.open_context(worker) for p in portals.values()
-                    ),
-                  # ^^NOTE XXX ^^^
-                    # it's the use of inline-generator syntax here
-                    # that causes the "empty input" -> ValueError,
-                    # see `._clustering` impl.
-                ),
+                gather_contexts(mngrs=()),
             ):
+                # should fail before this?
+                assert portals
+
                 # test should fail if we mk it here!
                 assert 0, 'Should have raised val-err !?'
 

tractor/_clustering.py

@@ -62,7 +62,7 @@ async def open_actor_cluster(
         ) as an
     ):
         async with (
-            tractor.trionics.collapse_eg(),
+            # tractor.trionics.collapse_eg(),
             trio.open_nursery() as tn,
             tractor.trionics.maybe_raise_from_masking_exc()
         ):