jstestnode: fix regression from introduction of Xvfb promise

${xvfb-instance:display} is used in in runTestSuite.in
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

09:39:06

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:43:39