test: refactor test-vm-sigint

* Use common.mustNotCall() to confirm SIGINT listeners are not being
  invoked.
* Improve assertion check on integer child argument.
* Add blank line per test writing guide.

PR-URL: #13902
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-vm-sigint.js

@@ -1,5 +1,6 @@
 'use strict';
 const common = require('../common');
+
 const assert = require('assert');
 const vm = require('vm');
 
@@ -15,7 +16,7 @@ if (process.argv[2] === 'child') {
   const method = process.argv[3];
   const listeners = +process.argv[4];
   assert.ok(method);
-  assert.ok(typeof listeners, 'number');
+  assert.ok(Number.isInteger(listeners));
 
   const script = `process.send('${method}'); while(true) {}`;
   const args = method === 'runInContext' ?
@@ -24,7 +25,7 @@ if (process.argv[2] === 'child') {
   const options = { breakOnSigint: true };
 
   for (let i = 0; i < listeners; i++)
-    process.on('SIGINT', common.noop);
+    process.on('SIGINT', common.mustNotCall());
 
   assert.throws(() => { vm[method](script, ...args, options); },
                 /^Error: Script execution interrupted\.$/);