Unit test for BMD boundary cases

[sedavid01]

We want to ensure that BMD censoring is consistent across builds and is working as intended.

Goal: Generate a unit test to ensure that the BMD censoring function is working correctly. Cases to cover are below:

• [x] Estimated BMD is less than the lower 'threshold dose'.
• [x] Estimated BMD is greater than the upper 'threshold dose'
• [x] Estimated BMD is above the lower 'threshold dose' and below the 'reference dose' (lowest expt dose).
• [x] Estimated BMD is above the lower 'reference dose' (lowest expt. dose).
• [x] Estimated BMD is below the upper 'threshold dose' and above the 'reference dose' (highest expt dose).
• [x] Estimated BMD is below the upper 'reference dose' (highest expt dose).