test_preview: Fix incorrect wrapping
test_preview: Fix incorrect wrapping

The old test expected an incorrect wrapping due to the preview function
not using unicode properly, so it got the wrong length.

Signed-off-by: Johannes Löthberg <8398eb9892c5ab2ff439ca0b5e4b0706dd9ebef9@kyriasis.com>

TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse