tests: {successorssets} now always return a list, affecting json output

CORE-TEST-OUTPUT-UPDATE: f95b23283760
12 jobs for topic/default/compat-successorssets in 6 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed checks-py2 #112108

00:01:10

passed checks-py3 #112109

00:01:03

passed doc #112114

00:00:28

manual pytype #112107
allowed to fail manual
passed tests-py2-cext #112685

00:05:21

passed tests-py2-pure #112686

00:05:06

passed tests-py3-cext #112687

00:04:03

passed tests-py3-pure #112688

00:03:15

failed tests-py2-cext #112110

00:05:20

failed tests-py2-pure #112111

00:05:13

failed tests-py3-cext #112112

00:03:36

failed tests-py3-pure #112113

00:03:19