[Multi SDFG Branch, Fortran Frontend] Extract a piece of code block out of `create_sdfg_from_string()`, that simplifies the dependency graph found from `recursive_ast_improver()`.

[pratyai]

• Add a test helper class that can match the AST by simple patterns (simple := easy to describe, not brief or easy to parse without additional comments).
• Add a test tests/fortran/recursive_ast_improver_test.py that pretty exhaustively covers the two functions: recursive_ast_improver() and simplified_dependency_graph().