jnp.linalg.solve: fully implement batched cases

[jakevdp]

Some cases of batched solve previously led to errors in JAX but not in NumPy, e.g.

x = np.random.rand(2, 2, 2, 2)
y = np.random.rand(2)
jnp.linalg.solve(x, y)  # error
np.linalg.solve(x, y)  # success

This PR fixes this class of error, and adds some more comprehensive tests of potentially ambiguous broadcasting semantics (cf. data-apis/array-api#285)

Related: #19221