Minor patches to unittests

- Sets cmake environment property of CppInterOp unitests to LLVM include dir
- 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

unittests/CMakeLists.txt

@@ -33,6 +33,7 @@ endif()
   add_test(NAME cppinterop-${name} COMMAND ${name})
   set_tests_properties(cppinterop-${name} PROPERTIES
                        TIMEOUT "${TIMEOUT_VALUE}"
+                       ENVIRONMENT "CPLUS_INCLUDE_PATH=${CMAKE_BINARY_DIR}/etc"
                        LABELS
                        DEPENDS)
   # FIXME: Just call gtest_add_tests this function is available.

unittests/CppInterOp/ScopeReflectionTest.cpp

@@ -1022,7 +1022,7 @@ TEST(ScopeReflectionTest, IncludeVector) {
     #include <vector>
     #include <iostream>
   )";
-  Interp->process(code);
+  Interp->declare(code);
 }
 
 TEST(ScopeReflectionTest, GetOperator) {