Gracefully shut down integration master and slave

Use SIGINT instead of the default SIGTERM when signalling to shut down
the integration master and slave subprocesses.  This should let nyc
properly write the coverage report before it exits.

test/integration/index.js

@@ -82,12 +82,12 @@ after(async function() {
 	this.timeout(200000);
 	if (slaveProcess) {
 		console.log("Shutting down slave");
-		slaveProcess.kill();
+		slaveProcess.kill("SIGINT");
 		await events.once(slaveProcess, "exit");
 	}
 	if (masterProcess) {
 		console.log("Shutting down master");
-		masterProcess.kill();
+		masterProcess.kill("SIGINT");
 		await events.once(masterProcess, "exit");
 	}
 });