net: reduce TestSplice/big's memory usage

The old code used splice on a 2GB []byte when not in short mode, meaning
that running 'go test net' when one had 4GB or less free memory would
easily result in "out of memory" runtime panics.

Instead, use a much smaller size that is still big enough to not fit
into a single splice(2) syscall. The new size is just 5MB, so the test
uses a fraction of the memory it used to, and there's no longer a need
for a different size on short mode.

This also speeds up the test, which goes from ~1.23s to ~0.01s on my
laptop.

Fixes #26867.

Change-Id: Iae1daa5c0995b549f41992f44339be32ca1ee5e4
Reviewed-on: https://go-review.googlesource.com/128535


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Andrei Tudor Călin <mail@acln.ro>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>