Backport #28361.

We add a test for np.nonzero under multi-threading and make np.nonzero safe under the cpython free-threading build. We want to ensure concurrent invocations of the method on the same array do not corrupt the system. Correct results are not guaranteed:

• If the underlying data is changing, the non-zero indices are not well-defined
• If the underlying data is changing, we can get indices outside the size of the array due to a part of the return array not being initialized.

Also see #27519. An alternative approach would be to use locks to make the array read-only during the operation.