Test case generation produced unusable output if the module contains
more than one function. Also, it was unnecessarily scattered around
several places in the code.
This CL consolidates test case generation in one method in the fuzzer,
and supports multiple functions with different signatures.

R=ahaas@chromium.org

Change-Id: I8bea71b0d69bb69d8bbe50002c6c7616a0a1941b
Reviewed-on: https://chromium-review.googlesource.com/847515
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50400}