test: refactor plugin_test.go file and optimize code

- Remove the `TestSetPasswordAndKey` function from `plugin_test.go`

Signed-off-by: Bo-Yi.Wu <appleboy.tw@gmail.com>

plugin_test.go

@@ -51,23 +51,6 @@ func TestMissingSourceConfig(t *testing.T) {
 	assert.NotNil(t, err)
 }
 
-func TestSetPasswordAndKey(t *testing.T) {
-	plugin := Plugin{
-		Config: Config{
-			Host:     []string{"example.com"},
-			Username: "ubuntu",
-			Port:     "443",
-			Password: "1234",
-			Key:      "test",
-		},
-	}
-
-	err := plugin.Exec()
-
-	assert.NotNil(t, err)
-	assert.Equal(t, errSetPasswordandKey, err)
-}
-
 func TestTrimElement(t *testing.T) {
 	var input, result []string