fix multiple issues with t5300

fix multiple issues with t5300

First of all, trying to run 'git verify-pack' on packs  produced by
the tests using pack.packSizeLimit always failed.  After lots of digging
and head scratching, it turns out that the preceeding test simulating
a SHA1 collision did leave the repository quite confused, impacting
subsequent tests.

So let's move that destructive test last, and add tests to run
verify-pack on the output from those packSizeLimit tests to catch such
goofage.

Finally, group those packSizeLimit tests together.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>