Bug 1568555 [wpt PR 18046] - MathML: Add tests for padding / margin / border-width, a=testonly

Automatic update from web-platform-tests
MathML: Add tests for padding / margin / border-width (#18046)

These are very basic tests to check support for
w3c/mathml#14

Put in a dedicated folder since it's likely that we need a more lot tests.
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wpt-commits: b9480320a5775a8cbb28725246070273c20cdf83
wpt-pr: 18046

UltraBlame original commit: a00d932c57dc1b8797bae222238046e9f96b9ee8

Author: marco-c