rubocop: Disable RSpec/ContextWording for shared_context descs

- I couldn't figure out a way to start these with "when", "with" or
  "without" given where these are in cop descriptions. Three ignored
  things out of 150 problems is a good ratio though.

Library/Homebrew/test/commands_spec.rb

@@ -3,7 +3,7 @@
 
 require "commands"
 
-RSpec.shared_context "with custom internal commands" do
+RSpec.shared_context "custom internal commands" do # rubocop:disable RSpec/ContextWording
   let(:cmds) do
     [
       # internal commands

Library/Homebrew/test/support/helper/spec/shared_context/homebrew_cask.rb

@@ -31,7 +31,7 @@ module Cask
   end
 end
 
-RSpec.shared_context "Homebrew Cask", :needs_macos do
+RSpec.shared_context "Homebrew Cask", :needs_macos do # rubocop:disable RSpec/ContextWording
   around do |example|
     third_party_tap = Tap.fetch("third-party", "tap")
 

Library/Homebrew/test/support/helper/spec/shared_context/integration_test.rb

@@ -7,7 +7,7 @@ require "formula_installer"
 
 RSpec::Matchers.define_negated_matcher :be_a_failure, :be_a_success
 
-RSpec.shared_context "integration test" do
+RSpec.shared_context "integration test" do # rubocop:disable RSpec/ContextWording
   extend RSpec::Matchers::DSL
 
   matcher :be_a_success do