selftests/fsmount_ns: add missing TARGETS and fix cap test

selftests/fsmount_ns: add missing TARGETS and fix cap test

Add missing top-level kselftest TARGETS entries for empty_mntns and
fsmount_ns so that 'make kselftest' discovers and runs these tests.

Fix requires_cap_sys_admin test which always SKIPped because fsopen()
was called after enter_userns(), where CAP_SYS_ADMIN in the mount
namespace's user_ns is unavailable. Move fsopen/fsconfig before fork so
the configured fs_fd is inherited by the child, which then only needs to
call fsmount() after dropping privileges.

Fixes: 3ac7ea91f3d0 ("selftests: add FSMOUNT_NAMESPACE tests")
Signed-off-by: Christian Brauner <brauner@kernel.org>