Section 216(b) of the Fair Labor Standards Act authorizes group litigation by “similarly situated” employees. The phrase isn’t defined and district judges have applied “ad hoc tests of assorted rigor in assessing whether potential members are ‘similarly situated,’” the 5th U.S. Circuit Court of Appeals explained in an interlocutory appeal from a ruling that granted a request for “conditional certification” of a minimum-wage case as a collective action. The plaintiffs, four truck drivers who worked for KLLM Transport …