Testing first paragraph of musts in 2.8.2.3

Changed value to null from undefined.

Removed mutation events (split out into other file with async on
mutation observers?) and made all other indicated fixes.

Tests the namedItem() method on an HTMLOptionsCollection for correct
return type/value on single match, no match, and multiple match cases.

Moved from common/ to semantics/forms/the-select-element/ per feedback
from tobie. Tests should now satisfy ms2ger's code quality suggestions
and tobie's placement suggestions.