test: check memoryUsage properties

The properties on memoryUsage were not checked before,
this commit checks them.

PR-URL: #5546
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-memory-usage.js

@@ -3,4 +3,6 @@ require('../common');
 var assert = require('assert');
 
 var r = process.memoryUsage();
-assert.equal(true, r['rss'] > 0);
+assert.ok(r.rss > 0);
+assert.ok(r.heapTotal > 0);
+assert.ok(r.heapUsed > 0);