fix example Gpx 3d and functional test

[ftoromanoff]

There is a bug on the source_file_gpx_3d.html example:

• the gpx waypoints doesn't display
• console errors appear
  

As the functional test wasn't well design (we only test that the view is initialized even if the example add extra layer after that), it create error in the github action from time to time.
I change the functionnal test to see if the example is fully loaded and corrected in the example the source of the error.

The error is linked to the fact that a style.fill is needed (at least in parent.style) when we instanciate a new Style:

itowns/src/Core/Style.js

Line 508 in e56bf65

if (typeof this.fill.pattern == 'string') {

and the way we set the getter:

itowns/src/Core/Style.js

Line 144 in e56bf65

return style.parent[category][name] || defaultValue;

Or in this example the style is defined during the parsing.

This issue is solve in the PR #2135 as we remove the need for a fill.patern and in PR #2006 as we remove the definition of parent Style.

[AnthonyGlt]

Thanks for this MR :)