[1.x][Test] Fix console warning in field_editor.test.tsx (#682)

Currently, unit test field_editor.test.tsx shows a console warning:

```React.createElement: type is invalid -- expected a string
(for built-in components) or a class/function (for composite components)
but got: undefined. ```

This warning is because EuiCodeEditor is missing in the mock of elastic/eui,
which causes undefined EuiCodeEditor in the test.

PR Resolved:
#673

Backport PR:
#674

Signed-off-by: Anan Zhuang <ananzh@amazon.com>

src/plugins/index_pattern_management/public/components/field_editor/field_editor.test.tsx

@@ -45,6 +45,7 @@ jest.mock('@elastic/eui', () => ({
   EuiButtonEmpty: 'eui-button-empty',
   EuiCallOut: 'eui-call-out',
   EuiCode: 'eui-code',
+  EuiCodeEditor: 'eui-code-editor',
   EuiConfirmModal: 'eui-confirm-modal',
   EuiFieldNumber: 'eui-field-number',
   EuiFieldText: 'eui-field-text',