REF/TST: collect reduction tests
Similar to how to have tests.arithmetic for tests where we expect EA/Index/Series/DataFrame behavior to be analogous, this starts collecting the tests for reduction operations.  This is motivated by #24293.

This does not yet collect tests from tests.frame.test_analytics or any of tests.arrays or tests.indexes.

Optionally we may want parts of test_nanops to be part of test_reductions, since those functions will also be in the check-these-are-equivalent pile.

Parametrizes some tests in test_base.

Moved tests are unchanged, except for non-context usages of `pytest.raises` are context-ified; some tests are parametrized.