cmd/go/internal/modfetch/zip_sum_test: long test for zip sum stability
This CL adds a new test package which downloads specific versions of
~1000 modules in direct mode and verifies that modules have the same
sums and the zip files have the same SHA-256 hashes.
This test takes a long time to run and depends heavily on external
data that may disappear. It must be enabled manually with -zipsum.
Fixes #35290
Change-Id: Ic6959e685096e8b09cea291f19d5bd0255432284
Reviewed-on: https://go-review.googlesource.com/c/go/+/204838 Reviewed-by: Bryan C. Mills <bcmills@google.com>