Large test file generation for benchmarking

[zouharvi]

Since the current test files are very small and cargo bench rounds the runtime to 0.0s, I was thinking about running the test on larger files (notably on (1) very long lines, (2) lots of lines or (3) both). The issue is that nobody really wants to have 0.5GB files in the repository.

Since their content is rather arbitrary, maybe we could just generate random data (with fixed seed) and have them be generated only locally? A few lines of a Python script would do but maybe it'd be purer adding a simple binary crate to this package.

In all cases I'm not sure if there's an elegant way to link the dependency other than erroring on tests that the files were not found and that the user probably forgot to generate them.