Cleaned up test contract layout

[jalextowle]

Fixes: #405.

Cleaned up the directory structure by merging test/mocks with contracts/test

As a side benefit, we were able to remove Mock4626 in favor of using MockERC4626 everywhere.

[coveralls]

coverage: 97.54%. remained the same
when pulling 916cf0b on jalextowle/cleanup/reorganize-test-contracts
into 71c6996 on main.

[github-actions]

Hyperdrive Gas Benchmark

Benchmark suite	Current: 916cf0b	Previous: e3658bb	Deviation	Status
addLiquidity: min	755 gas	733 gas	3.0014%	🚨
addLiquidity: avg	54091 gas	54041 gas	0.0925%	🚨
addLiquidity: max	96738 gas	96720 gas	0.0186%	🚨
checkpoint: min	558 gas	558 gas	0%	🟰
checkpoint: avg	47628 gas	47628 gas	0%	🟰
checkpoint: max	99410 gas	99410 gas	0%	🟰
closeLong: min	755 gas	755 gas	0%	🟰
closeLong: avg	23563 gas	23498 gas	0.2766%	🚨
closeLong: max	114200 gas	114183 gas	0.0149%	🚨
closeShort: min	713 gas	802 gas	-11.0973%	✅
closeShort: avg	26443 gas	26634 gas	-0.7171%	✅
closeShort: max	112799 gas	112872 gas	-0.0647%	✅
initialize: min	706 gas	662 gas	6.6465%	🚨
initialize: avg	177371 gas	177327 gas	0.0248%	🚨
initialize: max	251632 gas	251633 gas	-0.0004%	✅
openLong: min	757 gas	757 gas	0%	🟰
openLong: avg	56971 gas	56962 gas	0.0158%	🚨
openLong: max	218431 gas	218431 gas	0%	🟰
openShort: min	712 gas	712 gas	0%	🟰
openShort: avg	56248 gas	56198 gas	0.0890%	🚨
openShort: max	217822 gas	217803 gas	0.0087%	🚨
removeLiquidity: min	777 gas	777 gas	0%	🟰
removeLiquidity: avg	76002 gas	75980 gas	0.0290%	🚨
removeLiquidity: max	203187 gas	203187 gas	0%	🟰

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

[jrhea]

This is great. Pretty sure this resolves #405. You should put that in the description is you agree