ipn/ipnlocal: add c2n method to get SSH username candidates

For control to fetch a list of Tailscale SSH username candidates to
filter against the Tailnet's SSH policy to present some valid
candidates to a user.

Updates #3802
Updates tailscale/corp#7007

Change-Id: I3dce57b7a35e66891d5e5572e13ae6ef3c898498
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/ssh_test.go

@@ -2,14 +2,18 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux
-// +build linux
+//go:build linux || (darwin && !ios)
+// +build linux darwin,!ios
 
 package ipnlocal
 
 import (
+	"encoding/json"
 	"reflect"
 	"testing"
+
+	"tailscale.com/tailcfg"
+	"tailscale.com/util/must"
 )
 
 func TestSSHKeyGen(t *testing.T) {
@@ -40,3 +44,17 @@ func TestSSHKeyGen(t *testing.T) {
 		t.Errorf("got different keys on second call")
 	}
 }
+
+type fakeSSHServer struct {
+	SSHServer
+}
+
+func TestGetSSHUsernames(t *testing.T) {
+	b := new(LocalBackend)
+	b.sshServer = fakeSSHServer{}
+	res, err := b.getSSHUsernames(new(tailcfg.C2NSSHUsernamesRequest))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got: %s", must.Get(json.Marshal(res)))
+}