[wasm][test] Turn EXPECT_LENGTH macros into method

This removes another two macros and introduces a templatized function
instead.
Note that there is only one instantiation per input length, not per
input.

R=titzer@chromium.org

Bug: v8:8562
Change-Id: I47ad274e68d26b962cbd582e90995d30b1d09d39
Reviewed-on: https://chromium-review.googlesource.com/c/1460938
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59605}