[solana] Fix bug dropping first checkpoint in castVote

[zpoken]

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[zpoken]

The error in the test is related to the fact that two instructions combined into one transaction still did not get into one block. This was unexpected for me.

[zpoken]

The test assumes that we will overwrite the first checkpoint after creating a new account checkpoint. I managed to do this during local testing. Checked several times. Apparently this is not guaranteed in the test.

[zpoken]

The explanation for the error in the test turns out to be this:
In the CreateCheckpoints instruction, we copy the checkpoint.timestamp from the last checkpoint of the previous account.
https://github.com/wormhole-foundation/multigov/blob/solana/fix/bug-dropping-first-checkpoint-in-castVote/solana/programs/staking/src/lib.rs#L146
So to overwrite the first checkpoint we need to add the last checkpoint and add the second checkpoint in the same block. In this case the second checkpoint will not be created. That is, we need to combine not two instructions, but three instructions into one transaction.