tests: fix allow to skip running the pool tests again

tests: fix allow to skip running the pool tests again

`pytest -m not pool` should allow to skip the pool test. However,
because of the attribute access at import time to define the test
marker, import failed as well.

Convert the marker to strings which will be used in a getattr by the
fixture. Extend async-to-sync to convert the pool classes names from the
string too.