How does a Rule 12(b)(6) motion to dismiss test?

A Rule 12(b)(6) motion to dismiss tests the sufficiency of the complaint. Navarro v. Block, 250 F.3d 729, 732 (9th Cir. 2001). However, "[w]hile a complaint attacked by a Rule 12(b)(6) motion to dismiss does not need detailed factual allegations, a plaintiff's obligation to provide the grounds of his entitlement to relief requires more than labels and conclusions, and a formulaic recitation of the elements of a cause of action will not do." Bell Atl. Corp. v. Twombly, 550 U.S. 544, 555 (2007) (internal quotation marks and citation omitted).