Adjust overflow threshold bound for pow tests.

The original bound was too weak and did result in some overflows causing our CI pipelines to fail (https://gitlab.com/libeigen/eigen_ci_cross_testing/-/pipelines/631839537).

In particular, it allowed int32_t pow(256, 4), which overflowed to 0 for integers, and to an inexact result for float. Here we adjust the bounds.