selftests: openat2: add RESOLVE_ conflict test

Now that we reject conflicting RESOLVE_ flags, add a selftest to avoid
regressions.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20201027235044.5240-3-cyphar@cyphar.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>