Always use a local test server (#4207)

Some commands didn't setup a local test server since they didn't need
it. Other commands didn't setup a local test server because Seth forgot.

Long story short, I kept seeing weird requests to my Vault server when I
ran tests, and that should never happen. This ensures all test requests
will go to a test Vault instance.

Benchmarks show this adds 0.4s to the command test suite.

command/policy_delete_test.go

@@ -51,7 +51,11 @@ func TestPolicyDeleteCommand_Run(t *testing.T) {
 			t.Run(tc.name, func(t *testing.T) {
 				t.Parallel()
 
+				client, closer := testVaultServer(t)
+				defer closer()
+
 				ui, cmd := testPolicyDeleteCommand(t)
+				cmd.client = client
 
 				code := cmd.Run(tc.args)
 				if code != tc.code {