test-validator: create EpochRewards at epoch 0

[2501babe]

Problem

the bpf stake program depends on the existence of EpochRewards to function, but this account does not exist until epoch 1. this means:

1. all stake operations fail on solana-test-validator in the first epoch
2. all stake operations would fail on a brand new cluster in the first epoch

im not sure if the singlenode and multinode demos start fresh on epoch 0 or if they get accounts from some known source or fast forward past the first epoch

Summary of Changes

right now this is just a demonstration of something that does fix the issue for 1. im soliciting feedback on whether to just do this for now or whether there is a good place to put this concept that fixes it for 1 and 2. i believe if i copy-paste the same code in main() in genesis/src/main.rs it should solve it for 2. but while i think a hacky solution for 1 is fine, a solution that covers 2 should be cleaner

perhaps there is a good place in GenesisConfig to put this? or maybe SysvarCache or Bank should init the account to a clean default? genesis/src/genesis_accounts.rs originally looked promising but appears to just be a mechanism that creates a bunch of stake accounts for pre-launch token holders

basically there are a lot of "plausible" places i could do this but im not sure if there is a known "good" place to do it

closes #4928

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.8%. Comparing base (4ebe3c0) to head (66624cc).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #6125     +/-   ##
=========================================
- Coverage    82.9%    82.8%   -0.1%     
=========================================
  Files         840      840             
  Lines      377977   377990     +13     
=========================================
- Hits       313382   313332     -50     
- Misses      64595    64658     +63     

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