Shift rules_for_testing into the test itself (pantsbuild#19752)

This was confusing me, because we don't actually use this plugin
in-repo, it's only for a test. So I moved it into the test.

This has the added benefit of having Pants code not depend on
`internal_plugin` code.

src/python/pants/engine/internals/BUILD

@@ -18,7 +18,6 @@ python_tests(
     name="scheduler_integration_test",
     sources=["scheduler_integration_test.py"],
     dependencies=[
-        "pants-plugins/internal_plugins/rules_for_testing",
         "testprojects/src/python:hello_directory",
     ],
     timeout=180,

src/python/pants/engine/internals/scheduler_integration_test.py

@@ -3,8 +3,9 @@
 
 import os
 from pathlib import Path
+from textwrap import dedent
 
-from pants.testutil.pants_integration_test import ensure_daemon, run_pants
+from pants.testutil.pants_integration_test import ensure_daemon, run_pants, setup_tmpdir
 from pants.util.contextutil import temporary_dir
 
 
@@ -27,14 +28,55 @@ def test_visualize_to():
 
 @ensure_daemon
 def test_graceful_termination(use_pantsd: bool) -> None:
-    result = run_pants(
-        [
-            "--backend-packages=['pants.backend.python', 'internal_plugins.rules_for_testing']",
-            "list-and-die-for-testing",
-            "testprojects/src/python/hello/greet:greet",
-        ],
-        use_pantsd=use_pantsd,
-    )
-    result.assert_failure()
-    assert result.stdout == "testprojects/src/python/hello/greet:greet\n"
-    assert result.exit_code == 42
+    sources = {
+        "in_repo_plugins/bender/register.py": dedent(
+            '''\
+            from pants.engine.addresses import Addresses
+            from pants.engine.console import Console
+            from pants.engine.goal import Goal, GoalSubsystem
+            from pants.engine.rules import QueryRule, collect_rules, goal_rule
+
+
+            class ListAndDieForTestingSubsystem(GoalSubsystem):
+                """A fast and deadly variant of `./pants list`."""
+
+                name = "list-and-die-for-testing"
+                help = "A fast and deadly variant of `./pants list`."
+
+
+            class ListAndDieForTesting(Goal):
+                subsystem_cls = ListAndDieForTestingSubsystem
+                environment_behavior = Goal.EnvironmentBehavior.LOCAL_ONLY
+
+
+            @goal_rule
+            def fast_list_and_die_for_testing(console: Console, addresses: Addresses) -> ListAndDieForTesting:
+                for address in addresses:
+                    console.print_stdout(address.spec)
+                return ListAndDieForTesting(exit_code=42)
+
+
+            def rules():
+                return [
+                    *collect_rules(),
+                    # NB: Would be unused otherwise.
+                    QueryRule(ListAndDieForTestingSubsystem, []),
+                ]
+            '''
+        ),
+        "in_repo_plugins/bender/__init__.py": "",
+    }
+
+    with setup_tmpdir(sources) as tmpdir:
+        result = run_pants(
+            [
+                f"--pythonpath=['{tmpdir}/in_repo_plugins']",
+                "--backend-packages=['pants.backend.python', 'bender']",
+                "list-and-die-for-testing",
+                "testprojects/src/python/hello/greet:greet",
+            ],
+            use_pantsd=use_pantsd,
+        )
+        result.assert_failure()
+        assert result.stdout == "testprojects/src/python/hello/greet:greet\n", result.stderr
+        assert result.exit_code == 42