test: fix get_element_tag_name test to expect lowercase tagname

[juliandescottes]

See w3c/webdriver#1956 & w3c/webdriver#1962

[juliandescottes]

Thanks for the quick review!

[whimboo]

I wonder if we should add another test which returns the tag name of a namespaced element like svg:g or similar.

[gsnedders]

I wonder if we should add another test which returns the tag name of a namespaced element like svg:g or similar.

And elements whose local name is not entirely ASCII lowercase (e.g., svg:linearGradient), or whose namespace prefix is not entirely ASCII lowercase (e.g., xmlns:SvG="http://www.w3.org/2000/svg").

Plus obvious edge-cases from the definition of tagName, for example HTML elements where the node document is not an HTML document. (Really we probably want to test all these cases in both HTML documents and non-HTML documents!)

[gsnedders]

https://github.com/web-platform-tests/wpt/blob/master/dom/nodes/Element-tagName.html may also be useful as inspiration here.