test: make two tests portable to Python 3.10 and free of a startup race

[fxinfo24]

Problem

requires-python = ">=3.10", but both tests below hard-coded 3.11-only behaviour, so neither could run on a supported interpreter. A second failure was a genuine race that could bite on any interpreter.

1. asyncio.timeout is 3.11+ (tests/bridge/test_repl_bridge.py)

test_pointer_mtime_task_fires_and_advances_updated_at_ms used async with asyncio.timeout(3). On 3.10 this is an AttributeError and the test cannot run at all.

Rewritten with asyncio.wait_for around the polling coroutine — the portable equivalent. The refreshed value is carried out of the closure through a dict holder; nonlocal cannot target a name that is only bound inside the enclosing try block.

2. SIGINT/SIGTERM startup race (tests/test_init_integration.py)

Three tests signalled their child process after a fixed 100–300 ms delay. That races interpreter startup: if the signal arrives before setup_graceful_shutdown() installs its handler, the default disposition terminates the child and it exits -SIGINT rather than the handled 130/143.

Measured directly:

delay before signal	exit code
0.1 s	-2 (default disposition)
0.3 s	130 (handler ran)

This reads as a product bug but is purely test timing — the child had not finished importing yet.

Fix

Replaced the guessed delay with a readiness handshake:

• The child prints __READY__ once its handlers are installed.
• _run_in_subprocess(..., wait_for_ready=True) blocks until that marker appears before signalling.

_drain_until_ready reads from the child's stdout using select under a deadline, so a child that dies during startup cannot hang the test. The new wait_for_ready parameter defaults to False, leaving existing callers unaffected.

Verification

Green on both interpreters, which is the point for a portability fix:

• Python 3.11 (CI's pinned version): 61 passed
• Python 3.10 (the declared floor): 61 passed

tests/test_init_integration.py was run three consecutive times to confirm the race is actually gone rather than merely unobserved.

[github-actions]

Test Results

     5 files   1 021 suites   20m 44s ⏱️
16 104 tests 16 082 ✅ 22 💤 0 ❌
32 178 runs  32 107 ✅ 71 💤 0 ❌

Results for commit 643ed9e.