Fix and run unit tests

[erizhang]

Fix unit test compilation by resolving a global variable multiple definition error and update cpputest submodule.

The shutdown_server function pointer was defined directly in src/html_ui/html_game.h, causing multiple definition linker errors when included in multiple compilation units. This PR corrects it by declaring shutdown_server as extern in the header and providing its definition in src/html_ui/html_game.c.

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[cursor]

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