getentropy tests: Update after commit 473681a1a506da
- Use GETENTROPY_MAX instead of hard-coding the value.
- Check for EINVAL instead of EIO
Fixes: 473681a1a506 ("libc: Fix getentropy POSIX 2024 conformance issues")
(cherry picked from commit c5056a3931b41a803a24b89400d38d5c5f843612)