ssh/agent: fix TestMalformedRequests
ssh/agent: fix TestMalformedRequests

Use a net.Pipe, rather than setting up a local TCP connection,
for testing malformed SSH requests, since we don't need the
complex intricacies of a real connection to test this protocol
behavior.

Fixes golang/go#50161

Change-Id: I5e46c42041ddd03b06725469216b82b801990e64
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/371874
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
Run-TryBot: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/crypto,golang/crypto,golang/crypto