Add spec for Style/RescueModifier

The following syntax changes in Ruby 2.7, so the offense highlight range
of `Style/RescueModifier` was changed.
ruby/ruby@53b3be5

For the role of `Style/RescueModifier`, it seems reasonable to have different
offense highlight ranges between Ruby 2.6 and 2.7 against these syntax.
So just adding a test should be enough I think.

spec/rubocop/cop/style/rescue_modifier_spec.rb

@@ -18,13 +18,20 @@
     RUBY
   end
 
-  it 'registers an offense for modifier rescue around parallel assignment' do
+  it 'registers an offense for modifier rescue around parallel assignment', :ruby26 do
     expect_offense(<<~RUBY)
       a, b = 1, 2 rescue nil
       ^^^^^^^^^^^^^^^^^^^^^^ Avoid using `rescue` in its modifier form.
     RUBY
   end
 
+  it 'registers an offense for modifier rescue around parallel assignment', :ruby27 do
+    expect_offense(<<~RUBY)
+      a, b = 1, 2 rescue nil
+             ^^^^^^^^^^^^^^^ Avoid using `rescue` in its modifier form.
+    RUBY
+  end
+
   it 'handles more complex expression with modifier rescue' do
     expect_offense(<<~RUBY)
       method1 or method2 rescue handle