When compiled as mempcpy, the return value is the end of the destination
buffer, thus it cannot be used to refer to the start of it.
(cherry picked from commit 9aaaab7c6e4176e61c59b0a63c6ba906d875dc0e)
* string/test-strlen.c: New file.
* string/test-string.h: New file.
* string/test-strcmp.c: New file.
* string/test-strchr.c: New file.
* string/test-strrchr.c: New file.
* string/test-strcpy.c: New file.
* string/test-stpcpy.c: New file.
* string/test-strncpy.c: New file.
* string/test-stpncpy.c: New file.
* string/test-strpbrk.c: New file.
* string/test-strcspn.c: New file.
* string/test-strspn.c: New file.
* string/test-strcat.c: New file.
* string/test-strncmp.c: New file.
* string/test-memchr.c: New file.
* string/test-memcmp.c: New file.
* string/test-memset.c: New file.
* string/test-memcpy.c: New file.
* string/test-mempcpy.c: New file.
* string/test-memmove.c: New file.
* string/Makefile (strop-tests): New variable.
(tests): Add strop-tests.
(distribute): Add test-string.h.