Use cling::Declare instead of Process for include tests

This hints towards a bug in cling::Process when we JIT two includes one after another like
```
std::string code = R"(#include <vector> \n #include <iostream)"
Interp->process(code);
```

Author: aaronj0

Diff for: interpreter/CppInterOp/unittests/CppInterOp/ScopeReflectionTest.cpp

@@ -953,7 +953,7 @@ TEST(ScopeReflectionTest, IncludeVector) {
     #include <vector>
     #include <iostream>
   )";
-  Interp->process(code);
+  Interp->declare(code);
 }
 
 TEST(ScopeReflectionTest, GetBinaryOperator) {