test: Fix mock function to call the real one

test: Fix mock function to call the real one

Technically, to call the real symbol open() associated with the
linker flag --wrap=open, it should be invoked via __real_open().
It is matter in some build environment. Without it, the mock function
__wrap_open() calls open(), which is call for __wrap_open() itself
recursively.

ASAN build blames it:

[ RUN      ] test_sys_get_str_p1
[  ERROR   ] --- No entries for symbol __wrap_open.
/home/abuild/rpmbuild/BUILD/libsyscommon-5.0.0/tests/test-mock.c:16:
     error: Could not get value to mock function __wrap_open
/home/abuild/rpmbuild/BUILD/libsyscommon-5.0.0/tests/test-mock.c:35:
     note: Previously returned mock value was declared here

In this case, the __wrap_open() is recursively called. Within the
recursive call, there is no more reserved value to get as it has
already been consumed up by the previous __wrap_open(), which is test
failure. Fixed open() to __real_open() in order not to recursively
call the __wrap_open() but to call the real open().

Change-Id: Ib53f13ed68f335929569ad8755e86a711a08e274
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>