Add a starter file for spec

Library/Homebrew/test/dev-cmd/extract_spec.rb

@@ -0,0 +1,13 @@
+describe "brew extract", :integration_test do
+  it "extracts the most recent formula version without version argument" do
+    path = Tap::TAP_DIRECTORY/"homebrew/homebrew-foo"
+    (path/"Formula").mkpath
+    target = Tap.from_path(path)
+    formula_file = setup_test_formula "foo"
+
+    expect { brew "extract", "foo", target.name }
+      .to be_a_success
+
+    expect(path/"Formula/foo@1.0.rb").to exist
+  end
+end