Add third_party exclusion to gcovr coverage generation

Co-authored-by: johnpatek <31934875+johnpatek@users.noreply.github.com>

Author: Copilot

test/CMakeLists.txt

@@ -70,6 +70,6 @@ if(mio.cover)
       DEPENDS ${COVER_TARGETS})
     add_custom_command(
       TARGET cover
-      COMMAND gcovr -r ${CMAKE_CURRENT_SOURCE_DIR}/.. -e ${CMAKE_CURRENT_SOURCE_DIR})
+      COMMAND gcovr -r ${CMAKE_CURRENT_SOURCE_DIR}/.. -e ${CMAKE_CURRENT_SOURCE_DIR} -e '${CMAKE_CURRENT_SOURCE_DIR}/../third_party/.*')
   endif()
 endif()