The tests generated by --wasm-fuzzer-gen-test did not encode the locals
of functions yet. This CL fixes that.
A bit of care has to be taken to ensure that the locals are generated
in exactly the same order as in the module generated by the fuzzer.
This requires calling {addLocals} several times.

R=ahaas@chromium.org

Change-Id: I95237b0baef0731b6c164fddc8f12fa6f478e220
Reviewed-on: https://chromium-review.googlesource.com/848832
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50402}