Added trivial integration tests for brew linkage.

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

@@ -0,0 +1,35 @@
+describe "brew linkage", :integration_test do
+  before do
+    setup_test_formula "testball"
+    (HOMEBREW_CELLAR/"testball/0.0.1/foo").mkpath
+  end
+
+  context "no cache" do
+    it "works when no arguments are provided" do
+      expect { brew "linkage" }
+        .to be_a_success
+        .and not_to_output.to_stdout
+        .and not_to_output.to_stderr
+    end
+
+    it "works when one argument is provided" do
+      expect { brew "linkage", "testball" }
+        .to be_a_success
+        .and not_to_output.to_stderr
+    end
+  end
+
+  context "cache" do
+    it "works when no arguments are provided" do
+      expect { brew "linkage", "--cached" }
+        .to be_a_success
+        .and not_to_output.to_stderr
+    end
+
+    it "works when one argument is provided" do
+      expect { brew "linkage", "--cached", "testball" }
+        .to be_a_success
+        .and not_to_output.to_stderr
+    end
+  end
+end