Add tests for CSS mode completion

Add tests for CSS mode completion

* test/lisp/textmodes/css-mode-tests.el (css-test-property-values)
(css-test-property-values-no-duplicates)
(css-test-value-class-lookup): Use `seq-sort'.
(css-mode-tests--completions): New helper function for retrieving CSS
completions.
(css-test-complete-bang-rule, scss-test-complete-bang-rule)
(css-test-complete-property-value, css-test-complete-pseudo-class)
(css-test-complete-pseudo-element, css-test-complete-at-rule)
(scss-test-complete-at-rule, css-test-complete-property)
(css-test-complete-selector, css-test-complete-nested-selector)
(scss-test-complete-nested-selector): New tests.