tempfork/sshtest/ssh: add fork of golang.org/x/crypto/ssh for testing only

This fork golang.org/x/crypto/ssh (at upstream x/crypto git rev e47973b1c1)
into tailscale.com/tempfork/sshtest/ssh so we can hack up the client in weird
ways to simulate other SSH clients seen in the wild.

Two changes were made to the files when they were copied from x/crypto:

* internal/poly1305 imports were replaced by the non-internal version;
  no code changes otherwise. It didn't need the internal one.
* all decode-with-passphrase funcs were deleted, to avoid
  using the internal package x/crypto/ssh/internal/bcrypt_pbkdf

Then the tests passed.

Updates #14969

Change-Id: Ibf1abebfe608c75fef4da0255314f65e54ce5077
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh_test.go

@@ -41,6 +41,7 @@ import (
 	"tailscale.com/sessionrecording"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tempfork/gliderlabs/ssh"
+	sshtest "tailscale.com/tempfork/sshtest/ssh"
 	"tailscale.com/tsd"
 	"tailscale.com/tstest"
 	"tailscale.com/types/key"
@@ -55,6 +56,8 @@ import (
 	"tailscale.com/wgengine"
 )
 
+type _ = sshtest.Client // TODO(bradfitz,percy): sshtest; delete this line
+
 func TestMatchRule(t *testing.T) {
 	someAction := new(tailcfg.SSHAction)
 	tests := []struct {