Fixes issue 3321 by ensuring labels are drawn as part of initial cons…

[macleodbroad-wf]

…truction of the DataAxis

Code Changes

• Resolves Script Error when clicking on a point in the Right Axis Example on visjs.org #3321
• Makes code creating svg elements testable by extending canvas-mock
• Adds unit tests to prevent future regression in this area

Tested by

• Verifying Example is fixed in Script Error when clicking on a point in the Right Axis Example on visjs.org #3321
• Verifying other graph2d examples behave the same as prior to the change.

R: @yotamberk