Add a runtime-aware test suite around `repl_fixture`, #385 follow-through

[goodboy]

Since the basic suite added in that patch-set (9ff448f) only audits
usage outside booting an actor, we need a pexpect-style suite to
also audit the feat within an actors context.

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Todo summary brought forward from #385

Compose an actor-runtime-aware suite (set) for all supported .devx
APIs pertaining to:

• varied, piece-wise tractor.pause/.pause_from_sync() + repl_fixture usage.
• use of the global ._state._runtime_vars['repl_fixture'] setting in some
  rent-actor and its subtree.
• verification that .devx.maybe_open_crash_handler() will enter when debug_mode=True
  as a globally modded default.

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(Testing) Todos from original (and now closed) gitea PR

• as repl_fixture input to sync code's use of [maybe_]_open_crash_handler()
  (which is currently implemented the same as the async APIs since
  async def post_mortem() just calls def _post_mortem())

  • the same but using the global rtvars override
    • which we should add an pub api for.. question is now or later?

• tests for all async uses which are already implemented via the
  augmentation of _pause()'s internal _enter_repl_sync()
  closure.

  • from root actor.
  • from any (nested) sub.
  • from an infected aio task?

• support for pause_from_sync()/breakpoint() ??

  • we need to break up the provided repl_fixture().__enter__()
    and .__exit__() calls with the latter invoked as part of the
    PdbREPL per method calls to DebugaStatus.release() to ensure
    the fixture's teardown isn't invoked until after the user
    releases their repl interaction.
    • likely impl will be to add an DebugStatus._fixture_stack: ExitStack and
      have every _pause() call .enter_context() prior to interaction and then
      have DebugStatus/PdbREPL call .fixture_stack.close()
      from .set_continue/quit()?

• support for an @acm impls of replx_fixture?

  • obvi this can only be supported from already async APIs or we
    have to somehow schedule the async code in the trio/runtime
    thread, so hardest to do for sync-code crashes/pauses.

Surrounding changes,

• maybe we can also include some (stringent) tests around similar usage of
  the undocumented Actor.lifetime_stack: ExitStack ??