[Tests] use `--legacy-peer-deps` for npm 7
[Tests] use `--legacy-peer-deps` for npm 7

This is needed because we test on eslint < 5, and `@typescript-eslint/parser` has peer dep requirements of eslint >= 5.

See https://github.com/ljharb/eslint-plugin-import/commit/b1aef78590f32b63d1539dac57141869ae039740

yannickcr/eslint-plugin-react