test: let curl show a potential error in silent mode

test: let curl show a potential error in silent mode

I collected a couple of fails in this particular test, but without any
output they're impossible to debug. Let's make this slightly less
annoying and let curl show an error (if any) even in silent mode.

This patch uncovers that curl has been (silently) complaining about not
being able to write to the output destination, because `grep -q`
short-circuits on the first match and doesn't bother reading the rest,
so replace `grep -q` with `grep ... >/dev/null` to force grep to always
read the whole thing from curl.