tests: allow specifying some nix config

Dependencies

• [2] 4WLW4VHS Test setup: support arbitrary hydra config
• [*] HX4QYOYA add first evaluations tests
• [*] UIA3ULNU Give each test its own Nix directories

Change contents

• edit in t/lib/Setup.pm at line 22
  [2.139]

  [2.139]
  + # * nix_config: text to include in the test's nix.conf
• edit in t/lib/Setup.pm at line 51
  [5.1878]

  [5.1878]
  + print $fh $opts{'nix_config'} || "";