1) Choose any tag or text
2) Choose Source -> Toggle Comment (or press Ctrl+Shift+C)
ASSERT: Line is commented
3) Make Undo (Ctrl+Z)
ASSERT: This action delete only the end of tag (-->)
4) Make Undo (Ctrl+Z) again
ASSERT: Delete start tag (<!--)

I think that the line should be uncommented from the first time of using Ctrl+Z

Another test case:
1) Select more than one line
2) Choose Source -> Add Block Comment
ASSERT: Block is commented
3) Choose Source -> Add Block Comment again
ASSERT: Nothing happens
4) Press Ctrl+Z (Undo)
ASSERT: New <!-- --> appeared around existed comments