tst_QXmlStream does not test non-wellformed documents properly

At first I noticed that the incremental parsing was not working because the file was already at its end, so we were not reading any data.

I created https://codereview.qt-project.org/c/qt/qtbase/+/636399 to address it.

Later I also noticed that the code that was feeding the data byte-by-byte was always skipping the first byte. So, all the provided documents were treated as non-wellformed.

After fixing it locally (see comment https://codereview.qt-project.org/c/qt/qtbase/+/636399/comment/5ab4c195_9d64cf2e/), I saw that six non-wellformed documents were treated as well-formed during the incremental parsing.

The scope of this task is to fix the test and the parser to make sure that the incemental parsing also works correctly.