Tests: Fix warning about unused variable

Resolve this warning:

 ".../Tests/CMakeCommands/target_link_libraries/depB.cpp", line 8: warning:
          variable "a" was declared but never referenced
    DepA a;
         ^

Tests/CMakeCommands/target_link_libraries/depB.cpp

@@ -7,5 +7,5 @@ int DepB::foo()
 {
   DepA a;
 
-  return 0;
+  return a.foo();
 }