Delete the cmd2_ext_test.py plugin code from the cmd2 repo

Also:
- Copy the ExternalTestMixin class implementation to the `tests_isolated_test_commandset/conftest.py` file
- Remove info on the External Test Plugin from the documentation
- Add info on the cmd2 Plugin Template to the documentation
- Remove loading of `cmd2-ext-test` source code from the pyproject.toml
- Removed ext-test stuff from tasks.py for invoke usage

There is likely a lot more cleanup that can and should be done. This is just a first attempt to get our GitHub Actions CI/CD working without this plugin's circular dependency.

Author: tleonhardt

docs/plugins/external_test.md

@@ -2,6 +2,6 @@
 
 <!--intro-start-->
 
-- [External Test Plugin](external_test.md)
+- [cmd2 Plugin Template](plugin_template.md)
 
 <!--intro-end-->

docs/plugins/index.md

@@ -0,0 +1,7 @@
+# cmd2 Plugin Template
+
+## Overview
+
+The [cmd2 Plugin Template](https://github.com/python-cmd2/cmd2/tree/main/plugins/template) is a
+general example that shows you how you can develop a plugin for `cmd2`. Plugins are generally mixin
+classes that add some extra functionality to your class which inherits from [cmd2.Cmd][].

docs/plugins/plugin_template.md

@@ -6,9 +6,10 @@ This covers special considerations when writing unit or integration tests for a
 
 ## Testing Commands
 
-The [External Test Plugin](plugins/external_test.md) provides a mixin class with a function that
-allows external calls to application commands. The `cmd2_ext_test.ExternalTestMixin.app_cmd`
-function captures and returns stdout, stderr, and the command-specific result data.
+We encourage `cmd2` application developers to look at the
+[cmd2 tests](https://github.com/python-cmd2/cmd2/tree/main/tests) for examples of how to perform
+unit and integration testing of `cmd2` commands. There are various helpers that will do things like
+capture and return stdout, stderr, and command-specific result data.
 
 ## Mocking
 

docs/testing.md

@@ -195,7 +195,7 @@ nav:
       - examples/examples.md
   - Plugins:
       - plugins/index.md
-      - plugins/external_test.md
+      - plugins/plugin_template.md
   - Testing:
       - testing.md
   - API Reference: