fitFfmDT: unit test test-VIF.R Error

Output message from pkg build below. Looks like an issue in passing the specFfm object, but this needs further debugging.

```
* checking tests ...
Running ‘tinytest.R’ ERROR
Running the tests in ‘tests/tinytest.R’ failed.
Complete output:
>
> if ( requireNamespace("tinytest", quietly=TRUE) ){
+ tinytest::test_package("FactorAnalytics")
+ }

test-VIF.R.................... 0 tests Error in `[.data.frame`(data, , .N, by = c(date.var, obj$exposures.char)) :
unused argument (by = c(date.var, obj$exposures.char))
Calls: ... lapply -> FUN -> eval -> eval -> fitFfm -> specFfm -> [
Execution halted
```