Revert "vp9: SVC unittests: fix to use y4m source."

Revert "vp9: SVC unittests: fix to use y4m source."

This reverts commit f0b491a52405abb1b3dbb6b2c74dd6a4c7a7ddb1.

This change results in unsigned integer overflows (as reported by
-fsanitize=integer) in datarate_test.cc,
for many of --gtest_filter=VP9/DatarateOnePassCbrSvc.OnePassCbrSvc*:
unsigned integer overflow: 167198 - 185560 cannot be represented in type
'unsigned long'

As the encoder didn't change, but the input with the change to
(correctly) use Y4mVideoSource, this revert is merely masking the issue.

BUG=webm:1352

Change-Id: Iecd9a6c83b3fca67c566732a5c92d36193cc2060