Resolve race condition in tests by keeping event loop alive

Add a dummy setTimeout() call that keeps the event loop alive
long enough to receive signals.
There may be better ways to do this, but this is simple and seems
sufficiently reliable.

test/fixtures/script.js

@@ -1,3 +1,6 @@
 #!/usr/bin/env node
 console.log('%j', process.execArgv)
 console.log('%j', process.argv.slice(2))
+
+// Keep the event loop alive long enough to receive signals.
+setTimeout(function() {}, 100)