svg-aam: test for implicit graphics-document role for svg elements

[jlp-craigmorten]

WPT is currently missing an assertion that a <svg> element has an implicit expected role of "graphics-document".

See relevant specifications:

• https://www.w3.org/TR/html-aria/#docconformance
• https://www.w3.org/TR/svg-aam-1.0/#details-id-66
• https://www.w3.org/TR/html-aam-1.0/#el-svg

Currently:

• The html-aam tests have a placeholder todo referencing the graphics-aam and svg-aam tests.
• The graphics-aam tests do not make this assertion as the specification does not currently provide role mappings.
• The svg-aam tests do not include an assertion on the <svg> element itself, only allowed children elements.
• The wai-aria tests do not include assertions on host language implicit roles.
• There are no explicit html-aria specification tests in WPT because it is focused on authors thus is ill-placed in WPT.

Proposal:

To ensure that the specifications are followed, an additional test asserting that an <svg> element has an implicit expected role of "graphics-document" in either:

• The html-aam tests adjacent to the placeholder referencing the graphics-aam and svg-aam tests.
• The svg-aam tests on the parent <svg> element that already exists in the test.

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Searched for existing tests using GitHub search, see https://github.com/search?q=repo%3Aweb-platform-tests%2Fwpt%20graphics-document&type=code. Please close if I've missed coverage somewhere!

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Relates somewhat to web-platform-tests/interop-accessibility#81