Flask brand API endpoint unit test
Move the os.environ and sys.path calls into a new setUpClass method instead of have them at the global level?
pass at the end of setUp should be removed, similarly the commented out print.
I'd put the constants for the URL and the headers into fields or methods (self.url() / self.url) to reduce duplication.
The docstrings are a bit much.  Ideally the code would be very self explanatory so that it almost spells out what you currently have (duplicated) in the docstring.
I'd put things that belong together in the same file; so same endpoint and requests definitely in the same file and probably in the same test class as well.  I'd go for practicality first - if you need to have convoluted setup because you want to handle many different tests, then split it into multiple classes such that the tests are again easy to read.