test: Verify specific CSS rules in DisplayHtml output

Previously, the test for <pre> styles was too broad, only checking for the existence of a <style> block without verifying its actual content.

This commit improves the tests to validate the specific CSS rules applied in the HTML generated by DisplayHtml.wrapMessageContent().

Changes include:
- Asserting that the correct font-family (monospace or sans-serif) is applied based on the htmlSettings.useFixedWidthFont value.
- Verifying specific rules for pre.k9mail: white-space, word-wrap, font-family, and margin-top.
- Checking that the .k9mail-signature class includes the opacity: 0.5; rule.