Change the test contracts names

[andreivladbrg]

I suggest the following changes:

• SablierV2UnitTest to SablierV2BaseTest
• SablierV2LinearUnitTest to SablierV2LinearTest
• SablierV2Linear__Foo to Foo_Tests

Example

SablierV2BaseTest is Test {}

SablierV2LinearBaseTest is SablierV2BaseTest {}

Foo_Tests is SablierV2LinearBaseTest {}

This way will be cleaner and will make our tests more DRY.

[andreivladbrg]

Addressed in this commit: 04b776d

[PaulRBerg]

@andreivladbrg I think that this has been addressed by #159.

Locking this discussion.