[icons] Bring back icon testid

[Janpot]

Avoid split brain between dev and production. It's too confusing. This brings back the testid, but allows for removing it with property legacyTestid. We can invert the default in v8.

Use the theme defaultProps to turn off this option globally:

const theme = createTheme({
  components: {
    SvgIcon: {
      defaultProps: {
        legacyTestid: false,
      },
    },
  },
});

Plan:

• v7.x: introduce the legacyTestid prop with default true to avoid breaking change.
• v8.0: make it default to false, the user must always explicitly turn it on in any environment.

[mui-bot]

Netlify deploy preview

https://deploy-preview-45766--material-ui.netlify.app/

Bundle size report

Bundle	Parsed size	Gzip size
@mui/material	🔺+492B(+0.09%)	🔺+233B(+0.15%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against d0c0a06

[p-hlp]

@Janpot We just updated to V7 and would love to have this merged. Is there any ETA :)?

[leonardocouy]

@Janpot would love to have this merged too. This in the only point missing to migrate from V6 to V7. Our big E2E suites are breaking due this.

[EtienneS76]

Any info on this issue ?
It's break auto-test on our apps to