tests/kernel/common: Fix dead code in sflist test

An inverted comparison typo led to the final loop in the sflist being
skipped.  Fix so that it actually runs.

(Odd that it took a static analysis tool to detect this, the loop
expressions are all constants, I'm surprised gcc didn't see it while
doing unrolling analysis).

Fixes #18437

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>