chore: deduplicate test files

[tserg]

What I did

There were some duplication with test_return_struct.py / test_return_tuple.py with test_struct_return.py / test_tuple_return.py in tests/functional/codegen/calling_convention. This PR merges the tests into the former, with some struct unit tests going into a new tests/functional/codegen/types/test_structs.py file.

How I did it

How to verify it

Commit message

chore: deduplicate test files.

This PR merges the cross-contract tests in `test_return_struct.py` / `test_return_tuple.py` 
with `test_struct_return.py` / `test_tuple_return.py` in  `tests/functional/codegen/calling_convention`. 
The remaining struct unit tests were moved into a new `tests/functional/codegen/types/test_structs.py`.

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (37ef8f4) 84.94% compared to head (9d166c1) 84.67%.

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3773      +/-   ##
==========================================
- Coverage   84.94%   84.67%   -0.28%     
==========================================
  Files          92       92              
  Lines       13594    13594              
  Branches     3060     3060              
==========================================
- Hits        11548    11511      -37     
- Misses       1560     1593      +33     
- Partials      486      490       +4     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.