tests: Limit automatic random seed to 8 digits.

tests: Limit automatic random seed to 8 digits.

In the event that the test_strtoumax can only convert 32-bit types for
whatever reason, attempting to pass values >= 2**32 spits an error.

We don't need this kind of excess range for the automatic random seed.
Just truncate it to 8 digits which will always fit in 32 bits.