[6.x] Fix Faker Unique caching issue

[deleugpn]

Fixes #32700

I tested this change in my own codebase and it worked. The idea is somewhat simple: Assuming Faker is a singleton, the Container factory is executed only once per test. That means within a test, Faker will do it's job of ensuring uniqueness whenever asked.
As soon as the test finishes and the container is flushed, Laravel will start a container from scratch, which will run the container factory once again and it will clear the unique cache from faker by calling $faker->unique(true) (as suggested #32700 (comment)).

The reason for this description is to pay extra attention in $app->singleton(). This is not something easy to catch on a unit test, after all we're testing if unique works across multiple tests. If somebody ever makes Faker a non-singleton, that would cause the factory to be executed on every instantiation of the class and would again break the unique().