Wrong np.isclose-rtol value used for test_fb_cases.py

[texadactyl]

Describe the bug

The numpy isclose() rtol parameter was too large. Fortunately, this fact did not impact the validity of previous test results since I was monitoring them manually. The test computations have been very close to historical results, given the amount of random noise introduced.

Additional context

Affected source files: test/fb_cases_util.py, test_fb_def.py