Fix test failures with upcoming file(1) 5.35. Thanks to Christoph Biedl for...

Fix test failures with upcoming file(1) 5.35. Thanks to Christoph Biedl for the heads-up in advance. (Closes: #912756)

The regexp now covers both libmagic < 5.35 and libmagic >= 5.35.

Signed-off-by: Chris Lamb <lamby@debian.org>