[Bug Report] 【el-table】 using <el-table> width <el-table-column type="selection" cell-class="testname">, the selection column is unindented

### Element UI version
2.15.14

### OS/Browsers version
Chrome120.0

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/armani-sh/pen/abxXXap

### Steps to reproduce
Create the table using and set the < el-table-Column > attribute " type: 'selection', cell-name: 'testname' "

### What is Expected?
The form selection column is indented normally

### What is actually happening?
The form selection column is unindented and aligned to the left