Our tests currently don't set {WasmModule::num_declared_functions}
correctly. This CL fixes that.
This enables the use of {WasmModule::num_declared_functions} instead of
{NativeModule::num_functions_ - NativeModule::num_imported_functions_}.

Drive-by: Fix {std::vector} reservation to reserve enough space for all
functions during decoding.

R=titzer@chromium.org

Change-Id: I6d7783aed1c0de3275fc72787dec17c38ff8c73b
Reviewed-on: https://chromium-review.googlesource.com/1106166
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53857}