Add SVG-specific "name from content" tests

[cookiecrook]

After https://github.com/w3c/svg-aam/issues/31 is resolved, see if there is more to do wrt item 3 which currently limits the "Name from Content" to "SVG text containers"… I think the spec language may be irrelevant (can be removed)…

Currently phrased as:

In step 2F, modify step iii by replacing:

For each child node of the current node:

with

If the element is a text container element, for each child node of the current node:

In any case, I think we can test SVG text containers, but the tests should probably wait on a working group resolution of https://github.com/w3c/svg-aam/issues/31 and then maybe add new file: /svg-aam/name/comp_name_from_content.html?

[cookiecrook]

This used to be part of #69 but may be blocked due to the open spec issue.