test: do an initial coverage capture

test: do an initial coverage capture

I noticed that our coverage reports miss some files completely - this
happens when the test doesn't touch the code in them at all, so the
generated coverage data (and resulting reports) have no information
about them. Let's fix this by doing an initial zero coverage capture
that contains a zeroed counter for every instrumented line in every
file, so when we later merge it with a capture from the test, it shows up
with a missing coverage instead of not showing at all.