[test] clean tests before making coverage report

Otherwise, compiler will not be run if tests have already been run, so
no coverage files and fail.
4 jobs for arvid@increase_coverage in 13 minutes and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #621771522
coq

00:11:11

20.01%
passed #621771520
coq:8.8

00:09:16

passed #621771521
coq:8.9

00:10:09

passed #621771523
test:docker

00:13:16