TEST: add test for compute_fp.py

[mmcky]

Coverage:

compute_fp.py                          176     92    48%

The coverage statistics here are largely driven by internal private functions used by compute_fixed_point. Should we:

• Add tests for private functions used by compute_fixed_point, or
• add ignore statements given the are implicitly tested through compute_fixed_point

[mmcky]

Note: Code Can be excluded from coverage.py using

# pragma: no cover

for functions, branching etc.