Indirect calls rely on fixed signature indexes. Thus make test case
generation output the signatures exactly like they appear in the module.

R=ahaas@chromium.org

Change-Id: I80b088024da759ec87695363aeefb28685e1d704
Reviewed-on: https://chromium-review.googlesource.com/c/1350831Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57837}