Only add the Nix system feature to the test that requires it

Dependencies

• [2] 4Y3LBQYI Use system-features from the Nix conf in the default machine file
• [3] 4WLW4VHS Test setup: support arbitrary hydra config
• [4] 6HYCG3DC tests: allow specifying some nix config
• [5] UIA3ULNU Give each test its own Nix directories
• [*] HX4QYOYA add first evaluations tests

Change contents

• replacement in t/default-machine-file.t at line 5
  [2.195]→[2.195:218](∅→∅)
  − my %ctx = test_init();
  [2.195]

  [2.218]
  + my %ctx = test_init(

  + nix_config => q|

  + system-features = test-system-feature

  + |

  + );
• edit in t/lib/Setup.pm at line 51
  [3.1878]→[2.1661:1718](∅→∅)
  − print $fh "system-features = test-system-feature\n";