Combinatorial MultiToken _transferFrom unit test

[Padraic-O-Mhuiris]

This PR adds unit tests for MultiToken._transferFrom(). In addition it also provides some framework guidance on how "combinatorial" unit tests are to be constructed differing to the design in protocolv2 to better structure and reduce line count.

The tests are constructed with a fix sender from (Alice) and receiver to (Bob). Also all transfers contain a caller context which will either be Alice for a direct transfer or Eve for an approved transfer of which there are two kinds - a global approval isApprovedForAll and a perTokenApproval which is self-evident. Error cases involve an underflow on the non-infinite approvalm, the from balance updates and an overflow on to balance updates.
To note we may want to consider a more explicative error messaging especially in context of the non-infinite approval underflow which would be more considerate for frontends.

As stated, this PR should highlight an arguably cleaner approach to our combinatorial unit testing framework which we have used previously in protocolV2.
As shown, a combinatorial test should be 1 test per file and in so far as possible define 4 functions, __setup, __fail, __success and __log which ought to provide a template for test construction.

• __setup - teardown and setup of contract state in context of each test case
• __fail - validates if the test case ought to produce an error and attempts to catch that error. Must return a boolean so that the success phase can be short-circuited if an error is expected.
• __success - If applicable, event registration and state caching can be made before the call to the function under test. State validation follows.
• __log - logs test case values for debugging purposes

[coveralls]

Pull Request Test Coverage Report for Build 4186351439

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 180 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+3.3%) to 3.311%

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Files with Coverage Reduction	New Missed Lines	%
contracts/libraries/AssetId.sol	3	0%
contracts/AaveYieldSource.sol	11	0%
contracts/BondWrapper.sol	28	0%
contracts/Hyperdrive.sol	138	0%

Totals
Change from base Build 4175784911:	3.3%
Covered Lines:	15
Relevant Lines:	453

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💛 - Coveralls

[jalextowle]

I think you did a great job with this framework. It's a great addition to the repo. Aside from the other comments that I left, I have two requests. First, I'd appreciate using combinatorial instead of combi. Since every shell worth its salt has tab auto-completion for files, there isn't a compelling reason to use the abbreviation IMO. Second, I'd like to see more comments in the combinatorial test. Specifically in the functions that really matter (__fail and __success).

My comments should be addressed before merging, but I will approve since it LGTM once those comments are addressed.