libstdc++: Initialize base in test allocator's constructor

This fixes a warning from one of the test allocators:
warning: base class 'class std::allocator<__gnu_test::copy_tracker>' should be explicitly initialized in the copy constructor [-Wextra]

libstdc++-v3/ChangeLog:

	* testsuite/util/testsuite_allocator.h (tracker_allocator):
	Initialize base class in copy constructor.