Add Linux UHID backend test hooks

Expose test-only hooks for the Linux UHID/uinput backend and add fake-syscall support for unit tests. Introduces LIBVIRTUALHID_ENABLE_TEST_HOOKS when BUILD_TESTS is enabled in CMake, adds system_* wrappers (open/close/read/write/ioctl/poll/access) and ScopedLinuxTestSyscalls to allow scripted syscall overrides. Adds extensive test helpers and fake backend behaviors in uhid_backend.cpp and a new header uhid_backend_test_hooks.hpp with public test APIs, plus related test CMake and unit test additions, enabling backend testing without real device nodes.

Author: ReenigneArcher

src/CMakeLists.txt

@@ -18,6 +18,12 @@ if(LIBVIRTUALHID_USES_THREADS)
     target_sources(${PROJECT_NAME}
       PRIVATE
       "${CMAKE_CURRENT_SOURCE_DIR}/platform/linux/uhid_backend.cpp")
+    if(BUILD_TESTS)
+        # Expose private Linux backend helpers only to tests so coverage does not depend on real device nodes.
+        target_compile_definitions(${PROJECT_NAME}
+          PRIVATE
+          LIBVIRTUALHID_ENABLE_TEST_HOOKS=1)
+    endif()
     target_link_libraries(${PROJECT_NAME}
       PUBLIC
       Threads::Threads)