QTextCodec::availableCodecs() does not list all codecs when ICU is enabled

For a simple test-case listing all available codecs, see QTBUG-38496.

QTextCodec::availableCodecs() first checks if a list of codecs is cached. If not, it runs setup (qtextcodec.cpp@603) which is empty if ICU is used (qtextcodec.cpp@299). Then it adds all codecs returned by QIcuCodec::availableCodecs() (qtextcodec.cpp@613) and returns the list.
QIcuCodec::availableCodecs() only returns the ICU codecs, however, plus "TSCII" (qicucodec.cpp@405). Yet QIcuCodec::codecForNameUnlocked() may also return Qt's built-in codecs as per loadQtCodec() (qicucodec.cpp@506 and qicucodec.cpp@346).