Provide validators for testing

[lorenzwalthert]

Provide functions that can be used to directly compute the number of lines of each file in a git repo, e.g. via

compute_file_length(path = ".")

which can then be used to validate the parsing by comparing the computed number of lines per file with the actual number of lines per file in the repository.

validate_parsing_via_file_length(path = ".")

In principal formalise what has been done locally for dplyr.