net: don't check LookupHost error in TestLookupNullByte

net: don't check LookupHost error in TestLookupNullByte

net.LookupHost("foo\x00bar") may resolve successfully on some networks.
Reduce the scope of the test to check only that the call doesn't panic.

Also update the test comment to reference the relevant issue.

Fixes #37031
Updates #31597

Change-Id: If175deed8121625ef507598c6145e937ccffd89e
Reviewed-on: https://go-review.googlesource.com/c/go/+/217729
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>