Added 2 grammar-spelling-* tests and active-selection-005-manual test

[TalbotG]

2 tests testing a word with both a spelling error and a grammar error:
grammar-spelling-errors-001.html
grammar-spelling-errors-001-ref.html
grammar-spelling-errors-002.html
grammar-spelling-errors-002-ref.html

1 test testing ::selection highlight and
styling of css property caret-color:
active-selection-005-manual.html

[fantasai]

A few comments:

• Afaict it's not possible to have a visible caret in the middle of a highlighted word. So I'm not sure the active-selection-005 case is actually testable. We might need to skip the ::selection case, and only test caret-color for spelling and grammar errors.
• Afaict, the contenteditable element needs to be focused in order for the spelling error to be visible in Firefox and Chrome. We should do that with JS, and adjust the instructions to clarify that the contenteditable needs to be active for the test to work.
• Both 001 and 002 test that both the color and background of ::grammar-error is overridden by ::spelling-error. But we don't have a test for a combined result. If in 001 you move 'color: green' from ::spelling-error to ::grammar-error (and leave it unspecified for ::spelling-error), and in 002 you move 'background: yellow' from ::spelling-error to ::grammar-error (and leave it unspecified for ::spelling-error), then the tests will check that both ::spelling-error and ::grammar-error are correctly applied, and that ::spelling-error overrides ::grammar-error where there's a conflict (but not otherwise).

[TalbotG]

The initial commit in this PR was done before July 7th. Therefore, I must close this PR and create a new one (in a few min: ) ... which will have the improved grammar-spelling-errors-001.html and grammar-spelling-errors-002.html tests with their also-improved respective references.

active-selection-005-manual.html
will be replaced by
spelling-error-004-manual.html