Use jq in system tests inspecting JSON data

Use jq in system tests inspecting JSON data

Inspecting JSON data using grep is error-prone, overly lax in some ways,
overly strict in others, and neither accurate nor expressive.  Use jq
for inspecting JSON data in the "statschannel" and "synthfromdnssec"
system tests to address these deficiencies.

(cherry picked from commit b494e02761e9996a6619ed31a17ff0b6f32647eb)