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
8 jobs for topic/stable/sdist-size in 7 minutes and 45 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #207018

00:01:37

passed checks-py3 #207019

00:01:04

passed doc #207024

00:00:39

manual pytype #207017
allowed to fail manual
passed tests-py2-cext #207020

00:07:45

passed tests-py2-pure #207021

00:04:22

passed tests-py3-cext #207022

00:03:57

passed tests-py3-pure #207023

00:07:34