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