t5305: simplify packname handling

t5305: simplify packname handling

We generate a series of packfiles test-1-$pack,
test-2-$pack, with different properties and then examine
them. However we always store the packname generated by
pack-objects in the variable packname_1. This probably was
meant to be packname_2 in the second test, but it turns out
that it doesn't matter: once we are done with the first
pack, we can just keep using the same $packname variable.

So let's drop the confusing "_1" parameter. At the same
time, let's give test-1 and test-2 more descriptive names,
which can help keep them straight (note that we _could_
likewise overwrite the packfiles in each test, but by using
separate filenames, we are sure that test 2 does not
accidentally use the packfile from test 1).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>