test_retention: Update debugging code for CI failures.
test_retention: Update debugging code for CI failures.

This should provide more helpful output for the next stage of
debugging.

kou/zulip,synicalsyntax/zulip,shubhamdhama/zulip,showell/zulip,synicalsyntax/zulip,andersk/zulip,rht/zulip,timabbott/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,timabbott/zulip,brainwane/zulip,hackerkid/zulip,tommyip/zulip,zulip/zulip,timabbott/zulip,andersk/zulip,brainwane/zulip,rht/zulip,kou/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,shubhamdhama/zulip,andersk/zulip,zulip/zulip,kou/zulip,showell/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,synicalsyntax/zulip,punchagan/zulip,tommyip/zulip,shubhamdhama/zulip,eeshangarg/zulip,punchagan/zulip,brainwane/zulip,shubhamdhama/zulip,zulip/zulip,synicalsyntax/zulip,eeshangarg/zulip,rishig/zulip,zulip/zulip,timabbott/zulip,rishig/zulip,timabbott/zulip,rishig/zulip,punchagan/zulip,rht/zulip,kou/zulip,tommyip/zulip,shubhamdhama/zulip,brainwane/zulip,brainwane/zulip,tommyip/zulip,tommyip/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,timabbott/zulip,brainwane/zulip,showell/zulip,punchagan/zulip,synicalsyntax/zulip,eeshangarg/zulip,andersk/zulip,tommyip/zulip,rht/zulip,brainwane/zulip,hackerkid/zulip,rishig/zulip,andersk/zulip,shubhamdhama/zulip,hackerkid/zulip,punchagan/zulip,rishig/zulip,rishig/zulip,showell/zulip,synicalsyntax/zulip,kou/zulip,rht/zulip,tommyip/zulip,rishig/zulip,showell/zulip,rht/zulip,synicalsyntax/zulip,timabbott/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,shubhamdhama/zulip