Missing QTest::toString(q/uint128)

When Qt 6.6 added support for q(u)int128, it forgot to add QTest::toString() support for it. Without it, it's pretty hard to write tests using those types.

We already have a local implementation in tst_qglobal.cpp, just need to extract it, polish it, and somehow get it into tst_selftest.

TODO: Can we use qint128 under QT_SUPPORTS_INT128 protection, or do we require builds with and without __int128 support to be BC?

Acceptance criteria:

• QTest::toString() works with qint128 and quint128 on all QT_SUPPORTS_INT128 platforms
  • not by falling back on one of the narrower integer types
• checked by tst_selftest