stdlib: Tuned down tst-arc4random-thread internal parameters

stdlib: Tuned down tst-arc4random-thread internal parameters

With new arc4random implementation, the internal parameters might
require a lot of runtime and/or trigger some contention on older
kernels (which might trigger spurious timeout failures).

Also, since we are now testing getrandom entropy instead of an
userspace RNG, there is no much need to extensive testing.

With this change the tst-arc4random-thread goes from about 1m to
5s on a Ryzen 9 with 5.15.0-41-generic.

Checked on x86_64-linux-gnu.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>