CLN: Move stuff in tests.indexes.test_numeric.py to more logical locations

[topper-123]

This does four things to better organize the numeric index's tests:

1. Moves test method test_index_groupby from tests.indexes.test_numeric.py::TestNumericIndex to tests.indexes.common.py::Base and makes it use a fixture. This means this tests will be used for all index types. Also deletes TestNumericIndex, as its empty now.
2. Moves the Numeric class from test_numeric.py to common.py and renames it NumericBase.
3. Moves the tests.indexes.numeric.py::TestArithmetic::test_arithmetic_explicit_conversions tests to the new common.py::NumericBase and delete test_numeric.py::TestArithmetic class, as it's now empty.
4. Moves the file tests.indexes.test_numeric.py to tests.indexes.numeric.test_numeric.py.

[jreback]

looks fine, can you rebase.

@jbrockmendel ok here?

[jbrockmendel]

LGTM; needs rebase

[topper-123]

Ok, I’ve rebased.

[jreback]

thanks @topper-123