Convert `brew cat` test to spec.

Library/Homebrew/test/cat_test.rb

-require "testing_env"
-
-class IntegrationCommandTestCat < IntegrationCommandTestCase
-  def test_cat
-    formula_file = setup_test_formula "testball"
-    assert_equal formula_file.read.chomp, cmd("cat", "testball")
-  end
-end

Library/Homebrew/test/cmd/cat_spec.rb

+describe "brew cat", :integration_test do
+  it "prints the content of a given Formula" do
+    formula_file = setup_test_formula "testball"
+    content = formula_file.read
+
+    expect { brew "cat", "testball" }
+      .to output(content).to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+end