Modifiers for each testing branch

[PaulRBerg]

For this testing tree:

getSender.t.sol
├── when the stream does not exist
│  └── it should return zero
└── when the stream exists
   └── it should return the correct sender

Generate a test contract that uses modifiers to structure the tests:

contract SablierV2Linear__GetSender is SablierV2LinearUnitTest {
    /// @dev it should return zero.
    function testGetSender__StreamNonExistent() external {
        // ...
    }

    modifier StreamExistent() {
        _;
    }

    /// @dev it should return the correct sender.
    function testGetSender() external StreamExistent {
        // ...
    }
}

[PaulRBerg]

The long-term benefits of implementing this:

1. Makes it harder to fool yourself while writing tests. Because the test file mirrors the testing branch exactly, you can see very quickly what is the context and state for a given test function.
2. Reduces the amount of time spent thinking (and debating) how to name contracts and function names.
3. Reduces the information needed to provide in the test descriptors (the @dev NatSpec comments), since the contract name and inheritance structures provides the testing branch.
4. This is on objective way of generating tests based on a testing tree, so, in principle, it should be possible to write a script that takes a .tree file as an input and generates a skeleton file with the contract names and test functions.