Resolve coverage issues in `nox -s fortran_unit`

[dhermes]

It was here: https://github.com/dhermes/bezier/blob/429b56a465e9a05b37191f3eea0d926e14a1863f/.circleci/config.yml#L29-L31

It was removed in #246 because some combination of Ubuntu + gfortran + gcc + lcov updates has lead to phantom "missing" lines covered.

After running make lcov in tests/fortran, see one of the "missed" lines: