t/BuildDiff: slightly refactor empty test to be more readable

Dependencies

• [2] XJN33EVY BuildDiff: make actual hash ref
• [3] 3R3AYTCQ t/BuildDiff: init
• [4] BT5LSBCI t/BuildDiff: test diffing two jobs

Change contents

• replacement in t/Helper/BuildDiff.t at line 15
  [3.113]→[3.113:141](∅→∅)
  − subtest "response" => sub {
  [3.113]

  [3.141]
  + subtest "empty diff" => sub {
• replacement in t/Helper/BuildDiff.t at line 17
  [3.174]→[2.25:40](∅→∅),[2.40]→[3.189:404](∅→∅),[3.189]→[3.189:404](∅→∅),[3.404]→[2.41:49](∅→∅)
  − is($ret, {

  − stillSucceed => [],

  − stillFail => [],

  − nowSucceed => [],

  − nowFail => [],

  − new => [],

  − removed => [],

  − unfinished => [],

  − aborted => [],

  − failed => [],

  − });
  [3.174]

  [3.412]
  + is(

  + $ret,

  + {

  + stillSucceed => [],

  + stillFail => [],

  + nowSucceed => [],

  + nowFail => [],

  + new => [],

  + removed => [],

  + unfinished => [],

  + aborted => [],

  + failed => [],

  + },

  + "empty list of jobs returns empty diff"

  + );