t/helper: improve "genrandom" test helper
The `test-tool genrandom` test helper can be used to generate random data, either as an infinite stream or with a specified number of bytes. The way we handle parsing the number of bytes is lacking though: - We don't have good error handling, so if the caller for example uses `test-tool genrandom 200xyz` then we'll end up generating 200 bytes of random data successfully. - Many callers want to generate e.g. 1 kilobyte or megabyte of data, but they have to either use unwieldy numbers like 1048576, or they have to precompute them. Fix both of these issues by using `git_parse_ulong()` to parse the argument. This function has better error handling, and it knows to handle unit suffixes. Adapt a couple of our tests to use suffixes instead of manual computations. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>