Hit 95% of test coverage

[galachad]

We have some functions and files that are not cover by test coverage.

covr::package_coverage()
devtools::test_coverage()

cohortBuilder Coverage: 52.85%
R/attrition.R: 0.00%
R/cohortBuilder-package.R: 0.00%
R/repro_code_utils.R: 10.50%
R/hooks.R: 13.04%
R/list_operators.R: 36.67%
R/cohort_methods.R: 49.28%
R/source_tblist.R: 64.01%
R/filter.R: 67.43%
R/bind_keys.R: 77.27%
R/source_methods.R: 80.81%
R/step.R: 87.93%

All the functions should be cover with test.