libstdc++: Fix -Wmaybe-uninitialized warnings in testsuite

libstdc++: Fix -Wmaybe-uninitialized warnings in testsuite

These are false positives, but we might as well just value-init the
variables to avoid the warnings.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/allocator_traits/members/allocate_hint.cc:
	Value-initialize variables to avoid -Wmaybe-uninitialized
	warning.
	* testsuite/20_util/allocator_traits/members/allocate_hint_nonpod.cc:
	Likewise.
	* testsuite/20_util/duration/114244.cc: Likewise.
	* testsuite/20_util/duration/io.cc: Likewise.