Add specs for atom-text-editor shadow DOM selector upgrades

And cover one more case with `.editor` plus pseudo-classes

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Spy on console.warn to prevent spamming in the specs
Upgrade existing selectors after attaching atom-styles element

Style elements don’t have a .sheet property until they are actually
attached to the DOM.

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>