fix(process): register interrupt handler before raising in windows test

[dogancanbakir]

Fixes #767

TestSendInterrupt is flaky on windows-latest, and as written it is green only when SendInterrupt is too slow to matter.

The child branch raised the event with nothing watching SIGINT:

if os.Getenv("TEST_SEND_INTERRUPT_CHILD") == "1" {
    SendInterrupt()
    return
}

runtime.ctrlHandler maps CTRL_BREAK_EVENT to SIGINT but only consumes it when a handler is registered (if sigsend(s) { return 1 }). With no handler it returns 0 and Windows runs the default action — terminate with STATUS_CONTROL_C_EXIT (0xc000013a). Delivery is asynchronous, so it was a race between the handler thread and the child's return; the failing log shows the child emitting both PASS and exit status 0xc000013a.

This registers signal.Notify before raising and waits for the signal, matching what interrupt_unix_test.go already does. The race is gone (the handler consumes the event instead of the default action) and the test now asserts the thing it is named after. The parent-side re-exec with CREATE_NEW_PROCESS_GROUP is untouched — that is what keeps the break event away from sibling processes during go test ./....

GOOS=windows go vet ./process/ is clean; the behaviour itself needs the Windows runner to confirm, so watch this PR's Test Builds (windows-latest) job.