net: refactor readerAtEOF splice test

Refactor TestSplice/readerAtEOF to handle cases where we disable
splice on older kernels better.

If splice is disabled, net.splice and poll.Splice do not get to
observe EOF on the reader, because poll.Splice returns immediately
with EINVAL. The test fails unexpectedly, because the splice operation
is reported as not handled.

This change refactors the test to handle the aforementioned case
correctly, by not calling net.splice directly, but using a higher
level check.

Fixes #27355.

Change-Id: I0d5606b4775213f2dbbb84ef82ddfc3bab662a31
Reviewed-on: https://go-review.googlesource.com/132096


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>