cmd/tailscale: fix, test some recent doc inconsistencies

3dabea0fc2 added some docs with inconsistent usage docs.
This fixes them, and adds a test.

It also adds some other tests and fixes other verb tense
inconsistencies.

Updates tailscale/corp#25278

Change-Id: I94c2a8940791bddd7c35c1c3d5fb791a317370c2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/exitnode.go

@@ -41,7 +41,7 @@ func exitNodeCmd() *ffcli.Command {
 			{
 				Name:       "suggest",
 				ShortUsage: "tailscale exit-node suggest",
-				ShortHelp:  "Suggests the best available exit node",
+				ShortHelp:  "Suggest the best available exit node",
 				Exec:       runExitNodeSuggest,
 			}},
 			(func() []*ffcli.Command {