ci: disable node test runner process isolation to stop flaky failures

[dsherret]

Problem

The test:node task fails intermittently on CI (worst on macOS) with:

Error: Unable to deserialize cloned data due to invalid or unsupported version.
    at #processRawBuffer (node:internal/test_runner/runner:469:20)

Root cause

With the node test runner's default process isolation, node --test spawns the test file as a child process and parses the child's stdout pipe for v8-serialized test events (2-byte v8 header + 4-byte length + payload). Tests in this repo spawn real OS processes with stdio: "inherit", so those grandchildren write raw bytes directly into that same pipe, concurrently with the serializer. The parser tolerates raw bytes between frames (that's why the stray 5/1/2 lines normally print fine), but a write landing mid-frame corrupts the framing and fails the entire file. It's timing-dependent, hence flaky — and macOS is the worst case since pipe writes there are only atomic up to 512 bytes (PIPE_BUF), so serialized frames larger than that can be split with a grandchild's write interleaved in the gap.

Verified the mechanism with a repro: a test spawning a child that writes FF 0F-prefixed bytes to inherited stdout under node --test deterministically destroys the run. Matches upstream reports like nodejs/node#64061.

Fix

deno-test funnels every test file through a single runner file, so process isolation buys nothing here. --test-isolation=none (stable since Node 23; CI uses 24.x) runs the tests in the main process — no serialization pipe exists, and inherited stdio goes to the real terminal. Injected via NODE_OPTIONS since the deno-test bin doesn't forward extra flags.

Full suite passes locally with the flag, and the poison-byte repro passes cleanly too. Longer term, deno-test itself could pass --test-isolation=none since its single-runner design makes isolation pure overhead.