selftests/sched_ext: Fix enum resolution

All scx enums are now automatically generated from vmlinux.h and they
must be initialized using the SCX_ENUM_INIT() macro.

Fix the scx selftests to use this macro to properly initialize these
values.

Fixes: 8da7bf2c ("tools/sched_ext: Receive updates from SCX repo")
Reported-by: Ihor Solodrai <ihor.solodrai@pm.me>
Closes: https://lore.kernel.org/all/Z2tNK2oFDX1OPp8C@slm.duckdns.org/


Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>