Provide mock IOpipe for user function tests

[ewindisch]

Users developing with IOpipe will find that calls to context.iopipe.* will fail without a good mock. We should provide a reasonable mock for those users so they do not need to build this themselves.

[kolanos]

@ewindisch Can you provide an example of where context.iopipe.* fails? Just so I understand the use case.

[kolanos]

Closing s related to #285.