reftests: fix the linear-gradient reftest

reftests: fix the linear-gradient reftest

Don't use a repeating linear gradient, since it can't be easily
reftested against a non-repeating one for the reasons described in the
test header.
Instead, add a separate test for repeating gradients (against another
repeating gradient).

This makes the test pass, so it can be added to the Makefile now.