Add code generation test for CONSTANT block

[pramodk]

• code generation is already implemented in

  nmodl/src/codegen/codegen_helper_visitor.cpp

  Line 158 in 25cd470

  info.constant_variables = psymtab->get_variables_with_properties(NmodlType::constant_var);

  nmodl/src/codegen/codegen_cpp_visitor.cpp

  Line 2606 in 9602bdd

  for (const auto& var: info.constant_variables) {

• add test to make sure variables are added as part of mech_Store struct that contain all global variables

fixes #945

[bbpbuildbot]

Logfiles from GitLab pipeline #91506 (:white_check_mark:) have been uploaded here!

Status and direct links:

• ✅ spack_setup
• ✅ build:intel
• ✅ build:nvhpc
• ✅ test:intel
• ✅ test:nvhpc

[bbpbuildbot]

Logfiles from GitLab pipeline #93061 (:white_check_mark:) have been uploaded here!

Status and direct links:

• ✅ spack_setup
• ✅ build:intel
• ✅ build:nvhpc
• ✅ test:intel
• ✅ test:nvhpc