fix signal handling

[avtolstoy]

Problem

The runner ignores Ctrl-C, the terminal echoes ^C but tests keep running.

The gist: in Node, any SIGINT listener removes the default terminate behavior, and one that never calls process.exit() swallows the signal entirely. The runner loads all *.spec.js before applying filters, so a cleanup-only signal handler registered at module load by a shared helper (user/tests/integration/test/tethering.js) leaks into every run.

Solution

Adds InterruptHandler (lib/interrupt.js) - installs SIGINT/SIGTERM listeners before test files are loaded, runs best-effort cleanup (runner.shutdown()) on signal, then exits with the appropriate code. Second signal during cleanup exits immediately. Other listeners still run, so cleanup registered by test helpers is not lost.

[scott-brust]

No warning when building

Single ^C exits the test suite

➜  device-os git:(fix/gen3-usb-cdc-migrate-usbserial-tests) ✗ device-os-test --verbose --device-os-dir=. build wiring/no_fixture msom
Test directory: /Users/sbrust/develop/device-os/user/tests/integration
Device OS directory: /Users/sbrust/develop/device-os
Firmware binaries directory: /Users/sbrust/develop/device-os/build/integration
Target directory: /Users/sbrust/develop/device-os/build/integration
Building application: wiring/no_fixture
Platforms: msom
➜  device-os git:(fix/gen3-usb-cdc-migrate-usbserial-tests) ✗ device-os-test --verbose --device-os-dir=. run wiring/no_fixture msom
Test directory: /Users/sbrust/develop/device-os/user/tests/integration
Device OS directory: /Users/sbrust/develop/device-os
Firmware binaries directory: /Users/sbrust/develop/device-os/build/integration
Initializing API client
URL: https://api.particle.io
Authenticating with access token
Signed in as device-os-ci@particle.io
Subscribing to device events
Initializing device manager
Enumerating USB devices
Retrieving devices from the cloud
Device pool:
0a10aced202194944a04b000 (scott-us-msom-bg95m5-dvt1-03)
No fixtures configured
Generating test matrix
Running tests


  No fixture
    msom
      Target device: 0a10aced202194944a04b000 (scott-us-msom-bg95m5-dvt1-03)
      Flashing application: no_fixture.bin
      Getting device tests
      systemThread=disabled
          Initializing device test suite
          Running device test: APPLICATION_WATCHDOG_01_fires_timeout
          Device test passed
        ✓ APPLICATION_WATCHDOG_01_fires_timeout (4.9s)
          Running device test: APPLICATION_WATCHDOG_02_doesnt_fire_when_app_checks_in
^C
Received SIGINT, terminating