Provide a parameter to guarantee fixture uniqueness in `@fixture`

[smarie]

session-scoped fixtures are not guaranteed to be unique by session, and module-scoped fixtures are not guaranteed to be unique for an unique parameter, in pytest. See pytest-dev/pytest#2846

We could at least try to provide a workaround in the @cases_fixture decorator, such as ensure_unique=True.

Note that pytest-dev/pytest#3393 will not solve the problem because if users have two parametrized session-scoped fixtures, one of them will end-up being setup/teardown twice for each of its parameters.

[smarie]

Updated this issue since @cases_fixture is now deprecated and equivalent to @fixture + @parametrize_with_cases