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>