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Adds a fuzzer for the ImposedEnsembleMatcher. This achieves between
5000 and 10000 exec/s. At 10000 runs this covers 96% of the
imposed_ensemble_matcher and 50% of the io_utils (another file lacking
coverage). Uncovered lines in io_utils are attributed to debug tools.
The missing lines in imposed_ensemble_matcher are error cases which
haven't been hit yet. The seed uses duplicated back to back copies of
old.ztf and new.ztf.

Bug: 835341
Change-Id: I742ca6f4c409c9a9ec4a335da2b50fd8d4d6ed6f
Reviewed-on: https://chromium-review.googlesource.com/1117572
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572201}