Fix insufficient effective bal test and add a bal test

[terencechain]

This PR updated two things:

• Fixed the test_insufficient_effective_balance test. Although the effective balance was set to be less than 32 ETH, the actual balance was also set incorrectly because the balance needs to exceed 32 ETH plus the withdrawal amount. This PR added the withdrawal amount to the validator's balance to ensure the effective balance is the only condition triggering the failure
• Added a new test_insufficient_balance which has a sufficient effective balance but not enough actual balance, mimicking the above condition

[jtraglia]

Nice catch! Thank you!

[mkalinin]

New tests are very much appreciated! Thank you!