test suite: manpages-format: Cope with buster & eatmydata

test suite: manpages-format: Cope with buster & eatmydata

When man-db runs under eatmydata on buster, it (at least sometimes)
prints this message
  ERROR: ld.so: object 'libeatmydata.so' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.

(This is #963508 which I have just filed against glibc.)

Obviously it would be better if this just worked, but for now we can
definitely ignore this as it doesn't represent a bug in src:dgit's
manpages.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>