Fix encoder multi thread test failures
Fix encoder multi thread test failures

Caused by the prune_ref_frame_for_rect_partitions speed feature.

BUG=aomedia:2056

STATS_CHANGED

Change-Id: Ib43287fb66f8fca8b1482d301e841589692e847c

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom