Update two SVG tests to be consistent between browsers (#23240)

svg/import/pservers-grad-08-b-manual.svg:
This test renders some text using a font that doesn't contain the space glyph.
However, the content contains the space character, and doesn't specify a fallback
font. This means that different browsers on the same OS will pick different
fallback fonts, causing different renderings. Instead, the test should be updated
to specify a fallback font, which makes browsers render it consistently.

imported/w3c/web-platform-tests/svg/import/text-altglyph-01-b-manual.svg:
Similarly, this test renders text with a font that doesn't support those characters.
(It does it using <altGlyph>, but support for <altGlyph> has been removed in SVG 2.0
and neither Safari nor Firefox support it.) A fallback font should be supplied to the
test to get consistent renderings across browsers.

Co-authored-by: Myles C. Maxfield <mmaxfield@apple.com>