Adds getRate to MockHyperdriveTestnet

[jalextowle]

To make it possible to display the current variable rate in the internal trading competition, it needs to have access to the current yield source's rate.

[github-actions]

Hyperdrive Gas Benchmark

Benchmark suite	Current: 74588a7	Previous: 468e060	Deviation	Status
addLiquidity: min	785 gas	785 gas	0%	🟰
addLiquidity: avg	43558 gas	43700 gas	-0.3249%	✅
addLiquidity: max	80569 gas	80569 gas	0%	🟰
checkpoint: min	514 gas	514 gas	0%	🟰
checkpoint: avg	23044 gas	23044 gas	0%	🟰
checkpoint: max	33438 gas	33438 gas	0%	🟰
closeLong: min	852 gas	852 gas	0%	🟰
closeLong: avg	45875 gas	45736 gas	0.3039%	🚨
closeLong: max	86074 gas	77994 gas	10.3598%	🚨
closeShort: min	809 gas	809 gas	0%	🟰
closeShort: avg	41190 gas	40931 gas	0.6328%	🚨
closeShort: max	87306 gas	87306 gas	0%	🟰
initialize: min	714 gas	714 gas	0%	🟰
initialize: avg	159493 gas	159493 gas	0%	🟰
initialize: max	233625 gas	233625 gas	0%	🟰
openLong: min	740 gas	740 gas	0%	🟰
openLong: avg	114678 gas	114266 gas	0.3606%	🚨
openLong: max	180018 gas	180018 gas	0%	🟰
openShort: min	782 gas	782 gas	0%	🟰
openShort: avg	149276 gas	149518 gas	-0.1619%	✅
openShort: max	219306 gas	219306 gas	0%	🟰
removeLiquidity: min	762 gas	762 gas	0%	🟰
removeLiquidity: avg	57531 gas	57259 gas	0.4750%	🚨
removeLiquidity: max	119378 gas	119378 gas	0%	🟰

This comment was automatically generated by workflow using github-action-benchmark.

[coveralls]

coverage: 98.189%. remained the same when pulling 74588a7 on jalextowle/data/mock-hyperdrive-testnet-get-rate into 468e060 on main.