t/plugins/runcommand.t: output the filename if it fails to open

Dependencies

• [2] 3AKZKWCR RunCommand: Test
• [3] UWK5M3M6 runcommand.t: stop dumping the json
• [4] 2AM7IAS5 Tighten up 'should exit with return code'

Change contents

• replacement in t/plugins/runcommand.t at line 43
  [2.2521]→[2.2521:2570](∅→∅)
  − or die("Can't open \$filename\": $!\n");
  [2.2521]

  [2.2570]
  + or die("Can't open \"$filename\": $!\n");