[flang][openacc][NFC] Split global acc declare and function/subroutine declare

Split the tests for declare in module specification and the declare in
function/subroutine specificatons. Since the lowering is specific to the
location of the declare it will make it easier to update these files.
This also remove the duplicate parts shared by the FIR vs HLFIR tests.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D158222