Replaces use of Cop::investigate in favour of on_new_investigation

Replacing the deprecated Cop base class with Cop::Base means that we need to avoid
investigate. Following the example of comment processing Cops including with rubocop, we should
instead use on_new_investigation.

The expect_offense test helper also needed reworking. Cop::Base doesn't allow access to the offenses
detected. Those should come from the InvestigationReport from the Commissioner instance that invokes
the investigation (via Commissioner::investigate).

lib/smart_todo_cop.rb

@@ -15,7 +15,7 @@ class SmartTodoCop < Base
 
         # @param processed_source [RuboCop::ProcessedSource]
         # @return [void]
-        def investigate(processed_source)
+        def on_new_investigation
           processed_source.comments.each do |comment|
             next unless /^#\sTODO/.match?(comment.text)
 

test/smart_todo/smart_todo_cop_test.rb

@@ -84,19 +84,19 @@ def expected_message
 
     def expect_offense(source)
       annotated_source = RuboCop::RSpec::ExpectOffense::AnnotatedSource.parse(source)
-      investigate(annotated_source.plain_source)
+      report = investigate(annotated_source.plain_source)
 
-      actual_annotations = annotated_source.with_offense_annotations(cop.offenses)
-      assert_equal(actual_annotations.to_s, annotated_source.to_s)
+      actual_annotations = annotated_source.with_offense_annotations(report.offenses)
+      assert_equal(annotated_source.to_s, actual_annotations.to_s)
     end
     alias_method :expect_no_offense, :expect_offense
 
     def investigate(source, ruby_version = 2.5, file = "(file)")
       processed_source = RuboCop::ProcessedSource.new(source, ruby_version, file)
 
-      RuboCop::Cop::Commissioner.new([cop], [], raise_error: true).tap do |commissioner|
-        commissioner.investigate(processed_source)
-      end
+      assert(processed_source.valid_syntax?)
+      comm = RuboCop::Cop::Commissioner.new([cop], [], raise_error: true)
+      comm.investigate(processed_source)
     end
 
     def cop