[ArrayManager] TST: Enable extension tests
xref #39146

General note: I am using the `if hasattr(result._mgr, "blocks"):` to check whether we are using BlockManager or ArrayManager in the base extensions tests, to not rely on fixtures (like `using_array_manager`) that are defined higher up in pandas but are not available for downstream packages. @jreback @jbrockmendel any comments here? (trying to get all testing PRs merged to reduce the custom selection) Will take a look now. looks fine, may want to rebase