[openmp][flang][offloading] Do not use fixed device IDs in checks (#78973)

Fixes a small issues in an offloading test where the test dependec on
the host and device being assigned certains numeric IDs. This however is
not stable and fails in situations where any of the devices is assigned
an ID different from the expected value. The fix just checks that
offloading succeeded by making sure the IDs are different.

The test was failing locally for me.