PERF: Mark ImageRegistration8Test with RUNS_LONG

On the Rogue6 dashboard machine, even with the machine idle,
and running only one test at a time, it takes 515s.

Change-Id: I3438bdec88ce26a2360654127d91e0792b059c84
Merge topic 'ImageRegistration8Test-runs-long'

648bfdb4 PERF: Mark ImageRegistration8Test with RUNS_LONG