tests: improve tarball size check in test-check-sdist.t

Instead of relying on globs for checking numbers, let's rely on find(1)'s ability to check file size in a more intelligent way.

I would write the number as +800k, but it's not supported in some implementations of find, so it's not portable. Instead have to use "c" to make find(1) use bytes instead of 512-byte blocks, which is the default (???).

--HG-- branch : stable