parseBlackList() sheds line's capacity() d/t simplified() call

The parseBlackList() in src/testlib/qtestblacklist.cpp was ported to QIODevice::readLineInto() recently. This function is most effective when the following loop does not modify the arguments' capacity(). The one in parseBlackList() does (line = line.simplified(). Find out whether the simplified() is actually needed, or checkCondition() can be modified to treat arbitrary whitespace internally instead. Port the whole thing to QByteArrayView.

Reference: https://codereview.qt-project.org/c/qt/qtbase/+/582735/comment/fe8a41f7_e6476a86/