Added some tests for the Clock class.

test/unit.src.common.clock.spec.js

+const expect = require('chai').expect;
+const sinon = require('sinon');
+const Clock = require('../src/common/clock.js');
+
+describe("Clock Tests", function(){
+  var count = 0;
+  var tick = ()=>{count += 1;};
+  var clk = (new Clock()).clk(tick);
+
+  function speedTest(hz){
+    let stime = (new Date()).getTime();
+    let dt = 0;
+    clk.hz = hz;
+    while (count < hz && dt < 1000){
+      clk.tick();
+      dt = ((new Date()).getTime()) - stime;
+    }
+    return dt;
+  }
+
+  it("4hz for approx. 1 second", function(){
+    let dt = speedTest(4);
+    expect(count).to.equal(4);
+    expect(dt).to.be.gt(750);
+  });
+
+  it("4Khz for approx. 1 second", function(){
+    let dt = speedTest(4000); 
+    expect(count).to.be.lt(4004);
+    expect(dt).to.be.lt(1000);
+  });
+
+  it("4Mhz for approx. 1 second", function(){
+    let dt = speedTest(4000000); 
+    expect(count).to.be.lt(4000004);
+    expect(dt).to.be.lt(1000);
+  });
+});