[core] Add strict option to createMount

[eps1lon]

Adds an option to createMount that allows mount tests to run in strict mode.

const strictMount = createMount({ strict: true });
strictMount(<div />); // <React.StrictMode><div /></React.StrictMode>

const looseMount = createMount({ strict: false });
looseMount(<div />); // <React.Fragment><div /></React.Fragment>

const mount = createMount();
mount(<div />); // <div />

Why is strict: false different than strict: undefined?

If you still rely on childAt(0), dive etc. consistently adding an additional level to the component tree allows toggling strict mode without changing your test assertions.

own test suite changes

Runs every test that uses createMount either in strict mode or not.

Implementation

https://github.com/mui-org/material-ui/pull/15317/files#diff-b24ed23497b02191114acc4c33e82796

[mui-pr-bot]

No bundle size changes comparing 1e86a1c...be2e430

Generated by 🚫 dangerJS against be2e430

[eps1lon]

There were some very confusing fails. It's probably not strict mode ready at all.