Move string/testcopy.c to test-driver.c and xmalloc (bug 19667).

Bug 19667 reports unchecked malloc calls in the test
string/testcopy.c.  This patch makes that test use xmalloc and the
support/test-driver.c test framework.

Tested for x86_64.

	[BZ #19667]
	* string/testcopy.c: Include <support/support.h>.  Do not include
	<malloc.h>.  Use <support/test-driver.c>.
	(main): Rename to do_test.  Make static.  Use xmalloc instead of
	malloc.