Add more assets in invariant tests

[andreivladbrg]

Linked issues

• Closes https://github.com/sablier-labs/flow/issues/137
• Closes https://github.com/sablier-labs/flow/issues/157

Changes

• Add an array of assets in FlowCreateHandler
• Add useFuzzedAsset modifier in FlowCreateHandler
• Remove the unneeded Invariant_Test contract
• Add a helper function in FlowCreateHandler to prevent Stack too deep error
• Dry-fy the code by moving the test contracts in BaseHandler
• Run invariant_ContractBalanceGeStreamBalances test for all assets
• Use vm.assume instead of if(condition) return

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Note: I wanted to add the array of assets from test/invariant/Flow.t.sol to Base_Test, but I didn’t do it to prevent git conflicts.

[smol-ninja]

Note: I wanted to add the array of assets from test/invariant/Flow.t.sol to Base_Test, but I didn’t do it to prevent git conflicts

Unless, we are going to use this array in integration tests, I think its better to keep it in Flow contract.

[andreivladbrg]

@smol-ninja i've addressed your feedback, lmk if i missed anything