Add `open_crash_handler()` / `repl_fixture` suite

Nicely nailing 2 birds by leveraging the new `repl_fixture` support to
actually avoid use of a `pexpect`-style test B)

Functionality audit summary,
- ensures `open_crash_handler() as bxerr:` adheres to,
  - `raise_on_exit` semantics including only raising from a list of exc-types,
  - `repl_fixture` setup/teardown invocation and that `yield False` blocks REPL
    interaction,
  - delivering a `BoxedMaybeException` with the correct state set post
    crash.
  - all the above outside the actor-runtime existing.

Also luckily enough, this seems to have found a bug for which a fix is
coming right up!

tests/devx/test_tooling.py

@@ -14,6 +14,9 @@ TODO:
 
 '''
 from __future__ import annotations
+from contextlib import (
+    contextmanager as cm,
+)
 import os
 import signal
 import time
@@ -28,6 +31,8 @@ from .conftest import (
     PROMPT,
     _pause_msg,
 )
+
+import pytest
 from pexpect.exceptions import (
     # TIMEOUT,
     EOF,
@@ -183,3 +188,117 @@ def test_breakpoint_hook_restored(
     )
     child.sendline('c')
     child.expect(EOF)
+
+
+
+_to_raise = Exception('Triggering a crash')
+
+
+@pytest.mark.parametrize(
+    'to_raise',
+    [
+        None,
+        _to_raise,
+        RuntimeError('Never crash handle this!'),
+    ],
+)
+@pytest.mark.parametrize(
+    'raise_on_exit',
+    [
+        True,
+        [type(_to_raise)],
+        False,
+    ]
+)
+def test_crash_handler_cms(
+    debug_mode: bool,
+    to_raise: Exception,
+    raise_on_exit: bool|list[Exception],
+):
+    '''
+    Verify the `.devx.open_crash_handler()` API(s) by also
+    (conveniently enough) tesing its `repl_fixture: ContextManager`
+    param support which for this suite allows use to avoid use of
+    a `pexpect`-style-test since we use the fixture to avoid actually
+    entering `PdbpREPL.iteract()` :smirk:
+
+    '''
+    import tractor
+    # import trio
+
+    # state flags
+    repl_acquired: bool = False
+    repl_released: bool = False
+
+    @cm
+    def block_repl_ux(
+        repl: tractor.devx.debug.PdbREPL,
+        maybe_bxerr: (
+            tractor.devx._debug.BoxedMaybeException
+            |None
+        ) = None,
+        enter_repl: bool = True,
+
+    ) -> bool:
+        '''
+        Set pre/post-REPL state vars and bypass actual conole
+        interaction.
+
+        '''
+        nonlocal repl_acquired, repl_released
+
+        # task: trio.Task = trio.lowlevel.current_task()
+        # print(f'pre-REPL active_task={task.name}')
+
+        print('pre-REPL')
+        repl_acquired = True
+        yield False  # never actually .interact()
+        print('post-REPL')
+        repl_released = True
+
+    try:
+        # TODO, with runtime's `debug_mode` setting
+        # -[ ] need to open runtime tho obvi..
+        #
+        # with tractor.devx.maybe_open_crash_handler(
+        #     pdb=True,
+
+        with tractor.devx.open_crash_handler(
+            raise_on_exit=raise_on_exit,
+            repl_fixture=block_repl_ux
+        ) as bxerr:
+            if to_raise is not None:
+                raise to_raise
+
+    except Exception as _exc:
+        exc = _exc
+        if (
+            raise_on_exit is True
+            or
+            type(to_raise) in raise_on_exit
+        ):
+            assert (
+                exc
+                is
+                to_raise
+                is
+                bxerr.value
+            )
+
+        else:
+            raise
+    else:
+        assert (
+            to_raise is None
+            or
+            not raise_on_exit
+            or
+            type(to_raise) not in raise_on_exit
+        )
+        assert bxerr.value is to_raise
+
+    assert bxerr.raise_on_exit == raise_on_exit
+
+    if to_raise is not None:
+        assert repl_acquired
+        assert repl_released