This is done by adding a `isSuccessfullyExtracted` predicate that is
filled for primary files at the very end of the extractor invocation if
the frontend was performed successfully. If for example the extractor
crashes this will therefore not be filled.
The upgrade script is written so that `SuccessfullyExtractedFiles.ql`
on an upgraded script will give exactly the same results as before it.
In those kinds of tests the results may have different final classes
that are not necessarily visible (or tested) solely through the string
representation. For better testing and reading of expected results,
`getQlPrimaryClasses` is added in these cases.
