Add note about live_debug only working on javascript enabled specs

doc/development/testing_guide/best_practices.md

@@ -88,6 +88,8 @@ Finished in 34.51 seconds (files took 0.76702 seconds to load)
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+Note: `live_debug` only works on javascript enabled specs.
+
 ### `let` variables
 
 GitLab's RSpec suite has made extensive use of `let` variables to reduce