python how to split test files from code into separate directories

Below is a fizzbuzz program done TDD using unittest in one file, I run it in the command line using python fizzbuzz.py -v.

import unittest

class FizzBuzz():
    def fizz_buzz(self, number):
        if number % 3 == 0 and number % 5 == 0:
            return 'fizzbuzz'

        if number % 3 == 0:
            return 'fizz'

        if number % 5 == 0:
            return 'buzz'

        return str(number)

class FizzBuzzTests(unittest.TestCase):

    def setUp(self):
        self.fb = FizzBuzz()

    def test_one_gets_one(self):
        self.assertEqual('1', self.fb.fizz_buzz(1))

    def test_three_gets_fizz(self):
        self.assertEqual('fizz', self.fb.fizz_buzz(3))

    def test_five_gets_buzz(self):
        self.assertEqual('buzz', self.fb.fizz_buzz(5))

    def test_six_gets_fizz(self):
        self.assertEqual('fizz', self.fb.fizz_buzz(6))

    def test_fifteen_gets_fizzbuzz(self):
        self.assertEqual('fizzbuzz', self.fb.fizz_buzz(15))


if __name__ == '__main__':
    unittest.main()

I decided to split the file, as having the tests separate will read better.

The tree structure is

|-fizzbuzz
   |-app
     - __init__.py
     -fizzbuzz.py
   |-tests
     - __init__.py
     -test_fizzbuzz.py

I split the file as below (only showing part of it, one test and passing code)

#fizzbuzz.py
class FizzBuzz():
    def fizz_buzz(self, number):
        return str(number)

and

#test_fizzbuzz.py
import unittest
from fizzbuzz import FizzBuzz

class FizzBuzzTests(unittest.TestCase):

    def setUp(self):
        self.fb = FizzBuzz()

    def test_one_gets_one(self):
        self.assertEqual('1', self.fb.fizz_buzz(1))


if __name__ == '__main__':
    unittest.main()

In the fizzbuzz directory, i run tests with python tests/test_fizzbuzz.py which should pass.

But when I comment out the return str(number) or remove it, I still pass.

If I change self.assertEqual('1', self.fb.fizz_buzz(1)) to self.assertEqual('12', self.fb.fizz_buzz(1)) it is still running the code which is not there.

I deleted all the .pyc files in the test directory and now I get ImportError: No module named fizzbuzz error

If I move the fizzbuzz.py file into the test folder and run python tests/test_fizzbuzz.py the tests pass (this is how I got the .pyc files). But this is not what i want.

Why is this? Have I split this file up wrong? Am I missing extra code or files?

If I have split it up wrongly, what would be the best way to do so?

Any help appreciated, thanks.