Modify test.mk

1. The ethread test can only be ran when both the encoder and decoder are
available. This is also true for ext_tile test.
2. Modify the ethread speed test range to speed 0 to 3 for AV1.

BUG=aomedia:117

Change-Id: I3c3e96fb33a7b825503b228fe43e8089e1265dd2
2 files changed