Harden cluster tests

[goodboy]

In response to a weird issue discovered by @guilledk where if you pass in a mngrs=(<generator comprehension>), you end up with an empty sequence in the loop inside trionics.gather_contexts() and then a hang due to waiting on the all_entered: trio.Event just after the loop.

This solves the issue by greedly converting the mngrs sequence to a list and ensuring its size and otherwise raising a ValueError.

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Further TODO for this to land:

• [x] demonstrate issue through hanging test no commit for the fix yet pushed
  • exact test failure here in CI run from commit c606be8
• [x] push the actual fix (b5192cc)
• ~add tests for both the generator and empty list input cases~ there's no real reason for this, and further trying to jam in the logic in a pytest parametrization is janky..
• [x] nooz snippet

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Other additions part of this patch:

• we now passthrough all runtime kwargs to the open_actor_cluster() helper such that any kwarg to open_root_actor() can be used

[goodboy]

interesting i think 38326e8 makes the debugger tests pass that were failing on last run. I did a little local manual testing and it seems there can be transient failures on teardown..

I'm not sure if we should write an independent test to catch this since it's more of an auxillary teardown error that shouldn't (often) be a direct issue?