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/operator_unseal_test.go

@@ -27,7 +27,11 @@ func TestOperatorUnsealCommand_Run(t *testing.T) {
 	t.Run("error_non_terminal", func(t *testing.T) {
 		t.Parallel()
 
+		client, closer := testVaultServer(t)
+		defer closer()
+
 		ui, cmd := testOperatorUnsealCommand(t)
+		cmd.client = client
 		cmd.testOutput = ioutil.Discard
 
 		code := cmd.Run(nil)