In production, we will always pass a pointer to the decoded module. The
only reason for the existance of the nullptr checks is that tests
sometimes don't pass a module here. Hence we pay in production code for
a test-only feature.

This CL fixes this by always passing a module pointer from the decoder
tests. This even simplifies the code a lot by removing redundant code.

The {TestModuleBuilder} class was moved before the definitions of the
{FunctionBodyDecoderTest} class in order to be able to reuse it there.
It's unmodified otherwise.

R=ahaas@chromium.org

Bug: v8:10576
Change-Id: I7d876bfc8a27b50fe713afb8848fb4f642287cfa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2226749Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68116}