test/t/salsa.pm: use JSON

Johannes Schauer Marin Rodriguesrequested to merge
libjson-perl2 into master

libgitlab-api-v4-perl 0.27-1 switched from JSON to JSON::MaybeXS so we need to explicitly depend and load the JSON module

Closes: #1038486

This is an alternative to !355 (closed) which also closes this bug. This MR adds a new dependency on libjson-perl whereas !355 (closed) changes the codebase to do the same switch to JSON::MaybeXS as was done by libgitlab-api-v4-perl.

Thanks to @gregoa for the patch!

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