fix: correct mock epoch stake calculation

[abelmarnk]

Corrected the calculation of the remainder of the epoch stake from let remainder = target_total % num_accounts; to let remainder = target_total % base_stake;

[buffalojoec]

Hey thanks for the fix! Turns out this function is actually more broken than I thought.

Let target_total = 2.5 SOL (2,500,000,000).

num_accounts = 2_500_000_000.div_ceil(1_000_000_000) = 3
base_stake = 2_500_000_000 / 3 = 833_333_333
remainder = 2_500_000_000 % 3 = 1

That gives us three accounts:

1. 833_333_333 lamports
2. 833_333_333 lamports
3. 833_333_334 lamports

What it should actually be is this:

num_accounts = 2_500_000_000 / 1_000_000_000) = 2
base_stake = 2_500_000_000 / 2 = 1_000_000_000
remainder = 2_500_000_000 % 2 = 500_000_000

That gives us two accounts:

1. 1 SOL
2. 1.5 SOL

I pushed a commit to overhaul this and add some tests. Good spot!