[test] Add a default mount implementation to conformance tests

[eps1lon]

Allows us to remove usage of enzyme in most tests i.e. we remove the choice between enzyme and testing-library from most tests.

The goal is to make it straight forward to grep whether a tests needs enzyme (just search for createMount. mount is a bit too generic. Removing it from most tests is just better overall since it's one less concept people are confronted when reading a test.

Follow-up:

• Remove createMount where just need createPickerMount ([test] Use createPickerMount where appropriate #26951)
• Remove createMount call from tests that just need a wrapper component ([test] Only allow wrapping enzyme mount not creating #27018)
• Remove remaining mount usages where possible ([test] Use createPickerMount where appropriate #26951)

[mui-pr-bot]

No bundle size changes (experimental)

Generated by 🚫 dangerJS against 0c252e1

[oliviertassinari]

It will also make it harder for contributors to write test cases with mount: true story https://github.com/mui-org/material-ui/pull/26791/files#r655786565 😁

[eps1lon]

It will also make it harder for contributors to write test cases with mount: true story #26791 (files)

That's the goal, yes. In the end there will be all but 2 non-legacy tests remaining that will use enzyme.