fix man output in tests to not contain unprintable chars or terminal escape sequences

[joshrabinowitz]

See https://github.com/sobolevn/git-secret/pull/825#pullrequestreview-937323391 and https://github.com/sobolevn/git-secret/pull/825#issuecomment-1094415562

We tried using LC_CTYPE=C as suggested at https://unix.stackexchange.com/questions/497099/non-printable-characters-in-man-pages-and-logs but it didn't fix the problem.

I've piped man pages throught col -b in the past to strip out the unwanted characters but I'd rather find a solution which stops man from generating these sequences