tests: Ensure coverage data files get cleaned up at the end.
tests: Ensure coverage data files get cleaned up at the end.

Without calling cov.erase() the data file seems to persist and even
pollute future test runs if not removed. Registering an atexit handler
seems like a good, and reasonably clean way to ensure the cleanup
happens.
Fixes #13933.

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