Move at exit behavior into mainline SimpleCov not defaults

Naming it "custom" is slightly odd, as it's in the defaults
and if people wanted to differ they could define their own
at_exit hook and call whatever they wanted.

lib/minitest/simplecov_plugin.rb

@@ -3,7 +3,7 @@
 module Minitest
   def self.plugin_simplecov_init(_options)
     Minitest.after_run do
-      SimpleCov.custom_at_exit if SimpleCov.respond_to?(:custom_at_exit)
+      SimpleCov.at_exit_behvior if SimpleCov.respond_to?(:at_exit_behvior)
     end
   end
 end

lib/simplecov.rb

@@ -189,6 +189,14 @@ def exit_status_from_exception
       end
     end
 
+    def at_exit_behvior
+      # If we are in a different process than called start, don't interfere.
+      return if SimpleCov.pid != Process.pid
+
+      # If SimpleCov is no longer running then don't run exit tasks
+      SimpleCov.run_exit_tasks! if SimpleCov.running
+    end
+
     # @api private
     #
     # Called from at_exit block

lib/simplecov/defaults.rb

@@ -21,21 +21,11 @@
 # Gotta stash this a-s-a-p, see the CommandGuesser class and i.e. #110 for further info
 SimpleCov::CommandGuesser.original_run_command = "#{$PROGRAM_NAME} #{ARGV.join(' ')}"
 
-module SimpleCov
-  def self.custom_at_exit
-    # If we are in a different process than called start, don't interfere.
-    return if SimpleCov.pid != Process.pid
-
-    # If SimpleCov is no longer running then don't run exit tasks
-    SimpleCov.run_exit_tasks! if SimpleCov.running
-  end
-end
-
 at_exit do
   # Exit hook for Minitest defined in Minitest plugin
   next if defined?(Minitest)
 
-  SimpleCov.custom_at_exit
+  SimpleCov.at_exit_behvior
 end
 
 # Autoload config from ~/.simplecov if present