[Bug Report] Table's classname auto increased (e.g. el-table_2_column_6) caused jest snapshot test failed

### Element UI version
2.15.9

### OS/Browsers version
Jest test

### Vue version
2.7.0

### Reproduction Link
https://github.com/ElemeFE/element

### Steps to reproduce
In one jest test, test two component with same el-table render code, the snapshot result will be different between run them together and run them separately.

### What is Expected?
Is there any way I can reset tableIdSeed before each test?

### What is actually happening?
I can not reset tableIdSeed.