When `Chart.register()` was called twice in a row, the list of
plugin descriptors on the chart instance would be cleared. The
next chart update would then restart all of the plugins, not
knowing that they were already started. In the case of the Legend
and Title, this causes two boxes to appear in the layout system
thus drawing twice.
Cached plugin descriptors hold a reference on the plugin options, which break if the plugin options object is replaced. That case happens when the user updates the plugin options with a new object, but also since the new config update logic (#4198) that now always clones the plugin options. The fix consists in explicitly invalidating that cache before updating the chart.
`karma.conf.ci.js` has been merged into `karma.conf.js` for local testing consistency: `gulp unittestWatch` has been replaced by `gulp unittest --watch` and thus use exactly the same config file. Upgrade to latest jasmine and karma packages and remove deprecated `gulp-karma` dependency (directly use `karma.Server` in gulp).
Split `test/mockContext.js` into smaller `test/jasmine.*` modules to make easier unit tests maintenance and finally, move all `*.test.js` files under the `test/specs` folder.
