Tests: Code cleanup

[edg2s]

• Tests: Use ES6 in test-self/
• Tests: Use ES6 for RuleTester by default
• Tests: Avoid Array#concat

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1d557ed) to head (14ffe1f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #336   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          104       104           
  Lines          624       624           
=========================================
  Hits           624       624           

Files with missing lines	Coverage Δ
src/rules/no-event-shorthand.js	100.00% <ø> (ø)

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