LanguageServer/Cpp: Add tests

The Cpp LanguageServer tests can be run with: CppLanguageServer -t

The tests now only cover some very simple autocomplete and
"find declaration" use cases, but it's a start :)

Userland/DevTools/HackStudio/LanguageServers/Cpp/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(SOURCES
     LexerAutoComplete.cpp
     ParserAutoComplete.cpp
+    Tests.cpp
     main.cpp
 )