[Bug Report] El-collapse-item ellipsis overflow causes misaligned expand arrow

### Element UI version
2.15.1

### OS/Browsers version
Mac Catalina / Chrome 90

### Vue version
2.6.12

### Reproduction Link
https://jsfiddle.net/hb5oaywv/

### Steps to reproduce
Create an el-collapse-item with a very long title. Set the title to truncate overflows with ellipsis.

### What is Expected?
The > expand arrows should be aligned and not be rendered outside of the element width regardless of whether or not the title is truncated via ellipsis.

Element-ui version 2.4.9 does not exhibit this bug: https://jsfiddle.net/hb5oaywv/1/

### What is actually happening?
The > expand arrow on the right will be shifted to the right compared to non-truncated titles and rendering itself outside of the element's width.