test/unit/gui/proxy: Reset http(s) agent after reverting proxy
test/unit/gui/proxy: Reset http(s) agent after reverting proxy

  When reverting our proxy setup, we remove the global `http` and
  `https` agents, leading to errors with the new versions.
  Setting a new default agent after reverting the setup does the job.

cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop