[zip] Use ZipReader::Next() in tests

BUG=chromium:1295127
TEST=autoninja -C out/Default zlib_unittests && out/Default/zlib_unittests

Change-Id: Ic1048c46013190bf251f62b7e6dd82c065ea8298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3450064


Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#969788}
NOKEYCHECK=True
GitOrigin-RevId: 013178d0b5ccd034179d4da78129a225bb4fa359