PIFR should provide a way to test multisite installations.

Problem/Motivation

A patch was recently committed that broke all multisite installations due to the reversal of parameters to a newly-added function.

Proposed resolution

There should be some way to test basic multisite functionality so that this kind of trivial error doesn't sneak into core the next time.

Remaining tasks

A patch needs to be written, discussed, approved, and committed.

User interface changes

None.

API changes

New functions need to be added to DrupalTestCase and/or its subclasses to implement testing functionality.

The testing functionality needs to be supported by testbots.