tests: add test-check-sdist.t to check setup.py sdist results

For the record, we already have a somewhat similar test-check-setup-manifest.t,
but it only runs check-manifest and doesn't produce and check an actual tarball
in any way.

--HG--
branch : stable
8 jobs for topic/stable/manifest-new-py3 in 8 minutes and 42 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed checks-py2 #164601

00:01:03

passed checks-py3 #164602

00:00:53

passed doc #164607

00:00:16

manual pytype #164600
allowed to fail manual
passed tests-py2-cext #164603

00:03:50

passed tests-py2-pure #164604

00:03:15

passed tests-py3-cext #164605

00:04:01

passed tests-py3-pure #164606

00:03:38