selftests/nolibc: add sbrk_0 to test current brk getting

>From musl 0.9.14 (to the latest version 1.2.3), both sbrk() and brk()
have almost been disabled for they conflict with malloc, only sbrk(0) is
still permitted as a way to get the current location of the program
break, let's support such case.

EXPECT_PTRNE() is used to expect sbrk() always successfully getting the
current break.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>