Revert default mfrac's padding for mrow-fallback.html test (#47078)
This test compares the layout of invalid MathML markup and the one of an
<mrow> element. However, the UA style sheet adds padding around the
<mfrac> element by default while it doesn't for <mrow> [*].
Override this padding for the test, so that the size of an invalid
<mfrac> and the one of an <mrow> match perfectly.
[*] https://w3c.github.io/mathml-core/#user-agent-stylesheet