[RFC] Remove usage of popper.js's API

[oliviertassinari]

https://github.com/mui-org/material-ui-pickers/blob/b62ce2a71aa1395f02b7d54e11dbbfab8a080ea6/lib/src/_shared/PickerPopper.tsx#L70-L71

we rely on the API of popper.js's (onCreate), however, this leads to 3 issues:

1. It forces us to write async tests which should almost never be needed.

https://github.com/mui-org/material-ui-pickers/blob/d6134e05c71361f1a6c2d1ea210690e04d544c83/lib/src/__tests__/DateTimePickerTestingLib.test.tsx#L44

Without the wait for, it fails with:

2. It's unclear if this API will stay around in the future Popper changed home material-ui#19358
3. It could make it harder to expose the API to extern users.

I think that we could consider using a ref, to know when the DOM is safe to be usable

[oliviertassinari]

It seems it was already handled