This is useful for testing purposes so we can pass mocks to the
specific CodeFixProvider constructor.  For example 'add nuget reference'
will pass mocks for the nuget search and install services in its tests.

Also provide a way for code action operations to state if they should
be invoked during tests.  Lots of tests just want to examine the
state returned by certain operations (like SolutionChangedOperation).
However, for mutating operations, some tests need the operation to
actually apply so that they can check the results of the operation.