Fix simple RuboCop offenses and update TODO file (#1062)

* Fix Layout/EmptyLineAfterMagicComment offense

* Fix Layout/ExtraSpacing offense

* Fix Layout/ClosingParenthesisIndentation offenses

* Fix Style/MutableConstant offense

* Fix Style/UnneededInterpolation offenses

* Fix Style/RedundantParentheses offenses

* Update TODO config for RuboCop

* Add executable bit to test/test_helper.rb

ref: https://travis-ci.org/Shopify/liquid/jobs/488169512#L578

test/integration/error_handling_test.rb

@@ -123,7 +123,7 @@ class ErrorHandlingTest < Minitest::Test
             ',
       error_mode: :warn,
       line_numbers: true
-                                     )
+    )
 
     assert_equal ['Liquid syntax error (line 4): Unexpected character = in "1 =! 2"'],
       template.warnings.map(&:message)
@@ -140,7 +140,7 @@ class ErrorHandlingTest < Minitest::Test
                 ',
         error_mode: :strict,
         line_numbers: true
-                            )
+      )
     end
 
     assert_equal 'Liquid syntax error (line 4): Unexpected character = in "1 =! 2"', err.message
@@ -158,7 +158,7 @@ class ErrorHandlingTest < Minitest::Test
           bla
                 ',
         line_numbers: true
-                            )
+      )
     end
 
     assert_equal "Liquid syntax error (line 5): Unknown tag 'foo'", err.message