REGTESTS: use "command -v" instead of "which"

Ilya reported that the "which" utility is not that much portable and is
absent from Fedora. "type -p" is not portable either, and the correct
solution appears to be "command -v", so let's use this for now, we can
change it again in the future in case of problems.

Link: https://www.mail-archive.com/haproxy@formilux.org/msg36332.html
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