populate_db: Fix duplicate test data when run multithreaded.
populate_db: Fix duplicate test data when run multithreaded.

The previous implementation resulted in each of the 10 threads using
the same random seed and thus same messages.

Fixes #6028.

timabbott/zulip,shubhamdhama/zulip,rht/zulip,rht/zulip,timabbott/zulip,rishig/zulip,tommyip/zulip,brockwhittaker/zulip,shubhamdhama/zulip,verma-varsha/zulip,brainwane/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,brainwane/zulip,verma-varsha/zulip,vaidap/zulip,kou/zulip,verma-varsha/zulip,kou/zulip,tommyip/zulip,vaidap/zulip,shubhamdhama/zulip,brainwane/zulip,brockwhittaker/zulip,dhcrzf/zulip,andersk/zulip,eeshangarg/zulip,verma-varsha/zulip,rht/zulip,punchagan/zulip,Galexrt/zulip,Galexrt/zulip,mahim97/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,tommyip/zulip,jackrzhang/zulip,brainwane/zulip,brainwane/zulip,jackrzhang/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,synicalsyntax/zulip,Galexrt/zulip,kou/zulip,hackerkid/zulip,amanharitsh123/zulip,Galexrt/zulip,timabbott/zulip,tommyip/zulip,tommyip/zulip,showell/zulip,hackerkid/zulip,vaidap/zulip,amanharitsh123/zulip,jackrzhang/zulip,dhcrzf/zulip,synicalsyntax/zulip,dhcrzf/zulip,vaidap/zulip,amanharitsh123/zulip,showell/zulip,punchagan/zulip,Galexrt/zulip,zulip/zulip,andersk/zulip,synicalsyntax/zulip,rht/zulip,mahim97/zulip,shubhamdhama/zulip,eeshangarg/zulip,synicalsyntax/zulip,dhcrzf/zulip,zulip/zulip,showell/zulip,zulip/zulip,shubhamdhama/zulip,andersk/zulip,verma-varsha/zulip,rishig/zulip,mahim97/zulip,eeshangarg/zulip,zulip/zulip,timabbott/zulip,kou/zulip,dhcrzf/zulip,shubhamdhama/zulip,showell/zulip,rishig/zulip,showell/zulip,brockwhittaker/zulip,andersk/zulip,brockwhittaker/zulip,vaidap/zulip,kou/zulip,hackerkid/zulip,jackrzhang/zulip,Galexrt/zulip,timabbott/zulip,zulip/zulip,brainwane/zulip,hackerkid/zulip,rishig/zulip,jackrzhang/zulip,zulip/zulip,synicalsyntax/zulip,rishig/zulip,eeshangarg/zulip,synicalsyntax/zulip,jackrzhang/zulip,rishig/zulip,tommyip/zulip,brockwhittaker/zulip,amanharitsh123/zulip,Galexrt/zulip,punchagan/zulip,mahim97/zulip,rht/zulip,punchagan/zulip,rishig/zulip,shubhamdhama/zulip,vaidap/zulip,dhcrzf/zulip,jackrzhang/zulip,kou/zulip,mahim97/zulip,eeshangarg/zulip,showell/zulip,dhcrzf/zulip,andersk/zulip,mahim97/zulip,brockwhittaker/zulip,eeshangarg/zulip,andersk/zulip,verma-varsha/zulip,punchagan/zulip,timabbott/zulip,timabbott/zulip,amanharitsh123/zulip,zulip/zulip,tommyip/zulip,amanharitsh123/zulip,hackerkid/zulip,showell/zulip,kou/zulip