Add new process pending deposits test

[jtraglia]

This PR adds a new test which would have caught a bug in Prysm. From @terencechain:

I have a good spec test case that was triggered by devnet5 this morning and caused Prysm to fork off. The test case is relatively simple:

• There are two pending deposits in the state, both pointing to the same public key
• The public key does not exist in the beacon state
• The first pending deposit has an invalid signature, while the second has a valid signature

Pretty simple. This test should do exactly this.

I've also added a third deposit on Terence's advice:

If we want, we could add [third pending deposit] with the same amount or different amount and a make sure balance amount is correct

[mkalinin]

LGTM!