This method will be used for a test with multiple code spaces, to
encode large function indexes. The current implementation in
{wasmI32Const} just always uses 5 bytes for encoding the LEB value.
This CL adds a {wasmSignedLeb} function which properly encodes the
value, and adds tests for that.

Drive-by: Clean up the rest of {test-wasm-module-builder.js}.

R=mstarzinger@chromium.org

Bug: v8:9477
Change-Id: Ide2d90eed9d40aa28df680fbb413275346d9c0b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1725623Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62990}