docs: document how to test Flake8 plugins

Add plugin-development/testing-plugins.rst covering:
- editable install so entry points are visible
- legacy API: temp files + get_style_guide + check_files
- narrowing checks with select; full run without select
- subprocess CLI example with --select

Link the page from the plugin development toctree.

Closes #705.

Made-with: Cursor

docs/source/plugin-development/index.rst

@@ -43,6 +43,8 @@ section we cover:
 
 - How to write your check plugin so that it works with |Flake8| 2.x and 3.x
 
+- How to test your plugin (:doc:`testing-plugins`)
+
 
 Video Tutorial
 ==============
@@ -65,3 +67,4 @@ Detailed Plugin Development Documentation
     registering-plugins
     plugin-parameters
     formatters
+    testing-plugins