Fix poor test isolation in plugs_test.py (issue #753)

[harrish243]

Fixes #753

plugs_test.py had several class-level state leaks that made tests order-dependent and relied (in older versions) on a real socket being opened by FrontendAwareBasePlug via xmlrpcutil.SimpleThreadedXmlRpcServer.

Changes:

• Add module-level docstring explaining the hermetic design contract.
• Reset AdderPlug.LAST_INSTANCE in setUp() so each test starts with a clean slate (previously only INSTANCE_COUNT was reset, causing the LAST_INSTANCE reference from one test to bleed into the next).
• Reset TearDownRaisesPlug1.TORN_DOWN and TearDownRaisesPlug2.TORN_DOWN in setUp() so test_tear_down_raises cannot influence other tests via shared class-level boolean state.
• Add class-level and method-level docstrings that document why state is reset and confirm that no network socket is opened.
• Add a docstring to test_plug_updates() making it explicit that the update-notification path uses only in-process threading.Event objects (via SubscribableStateMixin) and no socket is involved.

All 9 existing tests continue to pass.

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