Element segments and tables in tests used an ad-hoc mechanism to
describe the different types of initializer expressions, e.g. an number
which could denote either the value of a constant or the index of a
global. This CL tidies up and generalizes the test infrastructure by
directly using WasmInitExpr in those cases.

Additional changes:
- Introduce WasmElemSegment class.
- Remove obsolete --experimental-wasm-bulk-memory flag from tests.
- Rename WasmInitExpr.type -> kind.
- Remove dependency of wasm-module-builder from mjsunit.js (except in
  assertTraps).

Change-Id: I716254a04ceea9ceb8ac6b848e12e1637f618f0d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2857638
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74368}