Getting a Table Formula in UNO

I'm trying to write a unit test for https://bugs.documentfoundation.org/show_bug.cgi?id=133647. As I see it, in order for this unit test to be effective, it needs to either compare to the formatted display value of the function (to check whether or not "** _expression_ is faulty **" is being shown) or compare to the formula text value (to see if it is getting translated the way I expect). But, I am having a heck of a time getting either of those in the unit test. 

I put what I have so far in https://paste.debian.net/1154267/ with the output at the bottom in a comment.

The operational code in DomainMapper_Impl.cxx appears to be writing the formula text to a com.sun.star.text.TextField when importing the document, but trying to get an XTextField from an XCell like this:
        uno::Reference<text::XTextField> xTextField(xCell, uno::UNO_QUERY);

In the unit test isn't working, xTextField.is() is false.

I can get a calculated value by getting an XText from the XCell, but that's not what I am after.

As far as looking at the formatted display value, from what I can tell in a debugging session it seems like I need DocumentFieldsManager::UpdateTableFields() to run, but I don't know if this is happening when running a unit test, maybe it isn't. I can't tell if there is some way to force it from a unit test.