Add missing line-height property to grid alignment tests.
The line-height property is recommended to be set for tests using the
Ahem font:
https://web-platform-tests.org/writing-tests/ahem.html
Differential Revision: https://phabricator.services.mozilla.com/D216060
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1906901
gecko-commit: 8ab8b2ade3847d82860006d0f9cf6c934b6a2aa8
gecko-reviewers: layout-reviewers, emilio