IndentingLineWriter not always correctly indent when carryover

When working on the QTBUG-117849, it was suggested to add some tests for carrying over lines when formatting https://codereview.qt-project.org/c/qt/qtdeclarative/+/537746/comment/63ce657c_81e07c33/

As at attempt to address this the corresponding test has been introduced https://codereview.qt-project.org/c/qt/qtdeclarative/+/540056/comment/6cdb406e_08b3820f/

However, it was found out that IndentingLineWriter is not always doing the work as expected.

For example, for the input "export const n1=1,n2=2,n3=3,n4=4,n5=5" it calculates the indent as equal to 0, hence, when the code is carried over, it starts writing it from the position 0.

The introduced test and `int FormatPartialStatus::indentLine()` might be a good starting point for the further investigation and fixing it.