TST: add tests for numpy.quantile (#23129)

This PR adds additional tests for quantiles:

1. Identification equation $E[V(q, Y)] = 0$
2. Adding a constant $c > 0$: $q(c + Y) = c + q(Y)$
3. Multiplying by a constant $c > 0$: $q(c \cdot Y) = c \cdot q(y)$
4. Multiplying by $-1$: $q_{\alpha}(-Y) = -q_{1-\alpha}(Y)$

(Note by seberg as reviewer: These tests are fairly complex, but may be useful for future development.  But if they seem too complicated, they are probably not really vital and could be shortened or removed.)

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