Refactor fixtures to allow for different scopes

[GeigerJ2]

Currently, many of the fixtures in AiiDA, such as aiida_localhost (here) are function-scoped (which is the pytest default). It would be nice to have a way to also use them at different scopes, e.g., class-scoped, for expensive setup-operations that should only be carried out once for a test class, for example.

One idea to allow this, originally brought up by @agoscinski on Slack, is to put the functionality currently contained in the fixtures into utility functions, which are then used by differently scoped fixtures, like:

# the logic that was in aiida_localhost is moved here
def utils_aiida_localhost(...):
    ...

# new fixture only class scope
@pytest.fixture(scope="class")
def aiida_localhost_class(...):
    utils_aiida_localhost(...)

# original fixture
@pytest.fixture(scope="function")  # Currently the default, even without explicitly specifying `scope="function"`
def aiida_localhost(...):  # or `aiida_localhost_function`, but backwards-incompatible
    utils_aiida_localhost(...)

To achieve this, a few other changes might also be necessary, as, e.g., tmp_path provided by pytest is also function scoped, so cannot be used in a class- or session-scoped fixture (instead, pytest provides the session-scoped tmp_path_factory fixture).