Document the use_external_destination_store test_init param

Dependencies

• [2] 6HYCG3DC tests: allow specifying some nix config
• [*] HX4QYOYA add first evaluations tests
• [*] 4WLW4VHS Test setup: support arbitrary hydra config

Change contents

• edit in t/lib/Setup.pm at line 23
  [2.56]

  [5.139]
  + # * use_external_destination_store: Boolean indicating whether hydra should

  + # use a destination store different from the evaluation store.

  + # True by default.