test(ci): coverpkg=gno.land/... for txtar tests

[moul]

Note: I'm uncertain about what will happen after the merge.

Fixes #3085
Addresses #3003

[gfanton]

I tried something more generic in 9adfc59 (#3086); unfortunately, it seems to makes the gnovm tests timeout :(

There is a chance that this PR could work since it only applies to gno.land, but having this kind of different rules per module/package will make potential similar issues, like the one you had in #3003, harder to debug in the futur.