WASHINGTON -- Lawsuits filed Thursday that challenge the federal government's approval of a Shell Oil Co. offshore exploration plan present a major test of regulators' power to swiftly review deep-water drilling blueprints. In two separate but overlapping filings, conservationists argue the government was bound by federal law to first finish a post-spill environmental study of the Gulf of Mexico before approving Shell's plan last month. The legal complaints were filed in the Atlanta-based 11th Circuit Court of Appeals — one by the Defenders of Wildlife, Center for Biological Diversity and the Natural Resources Defense Council and the other by Earthjustice,...