CppLanguageServer: Put cpp test files in /home/anon/cpp-tests/

This is similar to the LibJS test data that resides in
/home/anon/js-tests.
It's more convenient than storing the test programs as raw strings
in the code.

Userland/DevTools/HackStudio/LanguageServers/Cpp/Tests/find_variable_declaration.cpp

@@ -0,0 +1,4 @@
+int main(int argc, char** argv)
+{
+    argv = nullptr;
+}