MicNodeTransformerTest should be really improved to cover more cases

For example

```
testReplaceNodeByAnotherNode

	| root |
	root := Microdown parse: 'this is a paragraph'.
	self assert: root children first class equals: MicParagraphBlock.
	transformer visit: root.
	self assert: root children first class equals: MicTextBlock.
	self assert: root children first bodyString equals: 'test'
```

does not check the back pointer from the new children to their parent. 
