Add more testcases for class template argument deduction of maps

Add more testcases for class template argument deduction of maps

This adds additional tests for std::map and std::multimap CTAD. The
tests ensure that deduction works for braced-init-list of value_type
objects, and for pairs of input iterators (with both std::pair<Key, T>
and value_type as the iterator's value_type). This ensures deduction
from value_type still works, as well as the non-value_type cases in LWG
3025.

Similar tests for unordered maps do not work, apparently because the
constructor taking an initializer_list<value_type> is not usable for
deduction, and the deduction guide taking initializer_list<pair<Key, T>>
deduces key_type to be const. I am not addressing that.

	* testsuite/23_containers/map/cons/deduction.cc: Test deduction from
	initializer_list<value_type> and from input iterator ranges.
	* testsuite/23_containers/multimap/cons/deduction.cc: Likewise.

From-SVN: r267518