[test] Polish refactor

[oliviertassinari]

A continuation on #722, the changes:

• XGrid extends DataGrid. Tests that cover both should be at the DataGrid level.
• imports from @testing-library/react are not allowed, we proxy them all with test/utils. I have added an eslint rule to fail the CI and had to switch a test to strict: false, this test doesn't work in strict mode. It's important we know about it, the underlying logic of the grid is likey incorrect.
• Blank line convention between two different tests.
• We don't do ../../.., we use absolute imports from the root for simplicity.
• Move XGrid tests under a /tests folder for consistency with DataGrid.
• No empty space at the end of test block descriptions.
• We don't allow imports from test/utils/index in the main repo. I have added an eslint rule to fail the CI for consistency.
• Remove orphan async () => {.

Motived by seeing the same issues in #715, this PR should come before #715.