(PCP-650) Pin pcp-broker to 0.8.4 in tests
(PCP-650) Pin pcp-broker to 0.8.4 in tests

Pins pcp-broker to a specific version (0.8.4) during acceptance testing
to ensure reproducible tests and avoid breakage due to development on
pcp-broker.

[skip ci]

puppetlabs/pxp-agent,puppetlabs/pxp-agent,puppetlabs/pxp-agent