test/cmd: add more shell tests.

This should help ensure that these don't regress.

Library/Homebrew/test/cmd/casks_spec.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+RSpec.describe "brew casks", type: :system do
+  it "prints all installed Casks", :integration_test do
+    expect { brew_sh "casks" }
+      .to be_a_success
+      .and not_to_output.to_stderr
+  end
+end

Library/Homebrew/test/cmd/formulae_spec.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+RSpec.describe "brew formulae", type: :system do
+  it "prints all installed Formulae", :integration_test do
+    expect { brew_sh "formulae" }
+      .to be_a_success
+      .and not_to_output.to_stderr
+  end
+end

Library/Homebrew/test/cmd/list_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Homebrew::Cmd::List do
 
   it_behaves_like "parseable arguments"
 
-  it "prints all installed Formulae", :integration_test do
+  it "prints all installed formulae", :integration_test do
     formulae.each do |f|
       (HOMEBREW_CELLAR/f/"1.0/somedir").mkpath
     end
@@ -19,5 +19,9 @@ RSpec.describe Homebrew::Cmd::List do
       .and be_a_success
   end
 
-  # TODO: add a test for the shell fast-path (`brew_sh`)
+  it "prints all installed formulae and casks", :integration_test do
+    expect { brew_sh "list" }
+      .to be_a_success
+      .and not_to_output.to_stderr
+  end
 end

Library/Homebrew/test/cmd/setup-ruby_spec.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+RSpec.describe "brew setup-ruby", type: :system do
+  it "installs and configures Homebrew's Ruby", :integration_test do
+    expect { brew_sh "setup-ruby" }
+      .to output("").to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+end

Library/Homebrew/test/cmd/shellenv_spec.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+RSpec.describe "brew shellenv", type: :system do
+  it "prints export statements", :integration_test do
+    expect { brew_sh "shellenv" }
+      .to output(/.*/).to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+end