tests: mock __open_2()

tests: mock __open_2()

As of commit [1] glibc may overwrite a call to open() with call
to __open_2() (if only two arguments are provided and the code is
compiled with clang). But since we are not mocking the latter our
test suite is broken as tests try to access paths outside of our
repo.

1: https://sourceware.org/git/?p=glibc.git;a=commit;h=86889e22db329abac618c6a41f86c84657a15324
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>