libstdc++: Fix typos in iterator increment for std::text_encoding [PR117520]

libstdc++: Fix typos in iterator increment for std::text_encoding [PR117520]

The intended behaviour for std::text_encoding::aliases_view's iterator
is that it incrementing or decrementing too far sets it to a
value-initialized state, or fails an assertion when those are enabled.
There were typos that used == instead of = which meant that instead of
becoming singular or aborting, an out-of-range increment just did
nothing. This meant erroneous operations were well-defined and didn't
produce any undefined behaviour, but were not diagnosed with assertions
enabled, as had been intended.

This change fixes the bugs and adds more tests to verify the intended
behaviour.

libstdc++-v3/ChangeLog:

	PR libstdc++/117520
	* include/std/text_encoding (aliases_view:_Iterator::operator+=):
	Fix typos that caused == to be used instead of =.
	(aliases_view::_Iterator): Fix friend declaration.
	* testsuite/std/text_encoding/members.cc: Adjust expected
	behaviour of invalid subscript. Add tests for other erroneous
	operations on iterators.