Add input completion test suite (#4644)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

tests/CMakeLists.txt

@@ -26,6 +26,7 @@ set(test_SOURCES
     ${CMAKE_CURRENT_LIST_DIR}/src/Updates.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/Filters.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/LinkParser.cpp
+    ${CMAKE_CURRENT_LIST_DIR}/src/InputCompletion.cpp
     # Add your new file above this line!
     )