Update the test setup

With the latest rubocop-performance update, the tests are failing to run.
It looks like we need to update the setup.
Based off of rubocop/rubocop-minitest@a96a985.

test/cop_test.rb

@@ -11,13 +11,13 @@ def cop_class
   attr_reader :cop
 
   def setup
-    config = RuboCop::Config.new
-    @cop = cop_class.new(config)
+    @config = RuboCop::Config.new
+    @cop = cop_class.new(@config)
   end
 
   def investigate(cop, src, filename = nil)
     processed_source = RuboCop::ProcessedSource.new(src, RUBY_VERSION.to_f, filename)
-    team = RuboCop::Cop::Team.new([cop], nil, raise_error: true)
+    team = RuboCop::Cop::Team.new([cop], @config, raise_error: true)
     report = team.investigate(processed_source)
     report.offenses
   end

test/test_image_has_alt.rb

@@ -15,7 +15,7 @@ def test_image_has_alt_offense
     ERB
 
     assert_equal 1, offenses.count
-    assert_equal "Images should have an alt prop with meaningful text or an empty string for decorative images",
+    assert_equal "RailsAccessibility/ImageHasAlt: Images should have an alt prop with meaningful text or an empty string for decorative images",
                  offenses[0].message
   end
 

test/test_no_positive_tabindex.rb

@@ -15,7 +15,7 @@ def test_no_positive_tabindex_alt_offense
     ERB
 
     assert_equal 1, offenses.count
-    assert_equal "Positive tabindex is error-prone and often inaccessible.", offenses[0].message
+    assert_equal "RailsAccessibility/NoPositiveTabindex: Positive tabindex is error-prone and often inaccessible.", offenses[0].message
   end
 
   def test_no_positive_tabindex_alt_no_offense

test/test_no_redundant_image_alt.rb

@@ -15,7 +15,7 @@ def test_no_redundant_image_alt_offense
     ERB
 
     assert_equal 1, offenses.count
-    assert_equal "Alt prop should not contain `image` or `picture` as screen readers already announce the element as an image",
+    assert_equal "RailsAccessibility/NoRedundantImageAlt: Alt prop should not contain `image` or `picture` as screen readers already announce the element as an image",
                  offenses[0].message
   end