snappy: make TestFramingFormat not depend on downloading (separately licensed) testdata files.
snappy: make TestFramingFormat not depend on downloading (separately
licensed) testdata files.

Such files are now only used for benchmarks (they are the same files used
by the C++ snappy code).

LGTM=dsymonds
R=dsymonds
CC=golang-codereviews
https://codereview.appspot.com/201220043

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