Rewrite dict literal for files in tests/reshape/
- [ ] xref #38138 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Rewrites as dictionary literals for the following files:
 - ./pandas/tests/reshape/concat/test_datetimes.py
 -  ./pandas/tests/reshape/merge/test_merge.py
 -  ./pandas/tests/reshape/merge/test_multi.py