minor tweak to support running tests from within a tmux session

.rubocop.yml

@@ -505,6 +505,7 @@ Metrics/LineLength:
   - https
   Exclude:
     - lib/tmuxinator/window.rb
+    - lib/tmuxinator/cli.rb
     - spec/lib/tmuxinator/window_spec.rb
 Metrics/MethodLength:
   Description: Avoid methods longer than 10 lines of code.

spec/lib/tmuxinator/cli_spec.rb

@@ -286,7 +286,7 @@ describe Tmuxinator::Cli do
           allow(Tmuxinator::Config).to receive(:installed?).and_return(true)
         end
 
-        context "session exists" do
+        context "session exists", if: ENV["TMUX"].nil? do
           before(:all) do
             # Can't add variables through `let` in `before :all`.
             @session = "for-testing-tmuxinator"