img test: Don't rely on using an empty ImageMagick coder name

img test: Don't rely on using an empty ImageMagick coder name

This used to work, but has been interpreted as a literal part of
the filename since ImageMagick 6.9.8-3. In newer versions, there does
not seem to be any way to indicate that a filename containing ':' is
to be taken literally without first knowing the decoder to use.

Signed-off-by: Simon McVittie <smcv@debian.org>