Convert brew commands test to spec.

Library/Homebrew/test/cmd/commands_spec.rb

@@ -0,0 +1,7 @@
+describe "brew commands", :integration_test do
+  it "prints a list of all available commands" do
+    expect { brew "commands" }
+      .to output(/Built-in commands/).to_stdout
+      .and be_a_success
+  end
+end

Library/Homebrew/test/commands_test.rb

@@ -4,13 +4,6 @@ require "cmd/commands"
 require "fileutils"
 require "testing_env"
 
-class IntegrationCommandTestCommands < IntegrationCommandTestCase
-  def test_commands
-    assert_match "Built-in commands",
-                 cmd("commands")
-  end
-end
-
 class CommandsTests < Homebrew::TestCase
   def setup
     super