Extract `list_tests` from `Rails::TestUnit::Runner.load_tests`

This commit has been made for a future change
for Active Record bin/test to run its own database adapter related tests.

railties/lib/rails/test_unit/runner.rb

@@ -43,10 +43,7 @@ module Rails
         end
 
         def load_tests(argv)
-          patterns = extract_filters(argv)
-
-          tests = Rake::FileList[patterns.any? ? patterns : default_test_glob]
-          tests.exclude(default_test_exclude_glob) if patterns.empty?
+          tests = list_tests(argv)
           tests.to_a.each { |path| require File.expand_path(path) }
         end
 
@@ -94,6 +91,14 @@ module Rails
           def path_argument?(arg)
             %r"^[/\\]?\w+[/\\]".match?(arg)
           end
+
+          def list_tests(argv)
+            patterns = extract_filters(argv)
+
+            tests = Rake::FileList[patterns.any? ? patterns : default_test_glob]
+            tests.exclude(default_test_exclude_glob) if patterns.empty?
+            tests
+          end
       end
     end