(#19779) master_spec expects PSON::pretty_generate

The spec test for `puppet master --compile` now expects output to be formed
using `PSON::pretty_generate` instead of `Kernel.jj` which is susceptible to
monkey patching monkey business.
(#19779) Call PSON::pretty_generate instead of jj

The `jj` method can be clobbered if other JSON libraries, notably `json_pure`,
are loaded. When this occurs, `puppet master --compile` silently fails to
produce any output.