Staking Manager End-to-end tests

[cam-schultz]

Context and scope
Implement initial end-to-end tests exercising the staking manager contracts. In order to mark this issue as complete, the end-to-end tests should fully exercise the subnet staking contract side of the validator set management flows. Further work on the Avalanchego ACP 77 implementation is required before the end-to-end tests are able to manage the validator sets on the P-Chain.

Discussion and alternatives
We should cover at minimum the following cases:

• Adding a validator via initializeValidatorRegistration
• Removing a validator via initializeEndValidation
• Removing a validator via ExitValidatorTx

[geoff-vball]

Completed by #492