test.todo(title, fn) should produce a failure

[jamestalmage]

Overview:

My understanding of the skip vs todo vs failing API was:

1. test.skip([title], fn) - A test function is provided, but it is skipped(i.e. fn is never called). Common use: removing a failing test while you are in the middle of refactoring. People probably shouldn't be checking test.skip tests into their master branch (especially once test.failing is implemented). Title is optional, just like the normal test method.
2. test.todo(title) - No test function is allowed, but a title is required. Common use: documenting future tests (and maybe features) that need to be written.
3. test.failing([title], fn) - Proposed in A way to submit tests that should fail.  #673- Optional title, test function is required and will be run (but is expected to fail). Common use: allow users to create PR's that reproduce the problem. Reporting bugs in a way that does not break the build.

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Issue:

Right now the API for 2 is more like: test.todo(title, [fn]) - title is required, but fn is optional (and ignored).