[CN-Testing] Allow fixing of base allocation address

[ZippeyKeys12]

For pointer generation, we allocate a large buffer and generate addresses inside it (depending on ownership and size required).

This PR adds --fixed-alloc-base=<ADDR> which allows setting the address of this buffer.

[Copilot]

Pull request overview

Adds a new test-generation/runtime flag to pin the random input allocator’s backing buffer to a fixed virtual address, enabling reproducible pointer values across runs (especially when combined with a fixed --seed).

Changes:

• Add --fixed-alloc-base=<ADDR> CLI flag to cn-testing runner and OCaml test command.
• Implement fixed-address allocation via mmap (with MAP_FIXED_NOREPLACE where available) in bennet random allocator.
• Plumb the new option through test-generation config and build script generators (bash/make).

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
runtime/libcn/src/cn-testing/test.c	Parses and forwards --fixed-alloc-base to the random allocator.
runtime/libcn/src/bennet/state/rand_alloc.c	Implements fixed-base buffer mapping (mmap) and exposes a setter API.
runtime/libcn/include/bennet/state/rand_alloc.h	Documents and declares bennet_rand_alloc_set_fixed_base.
lib/testGeneration/testGenConfig.mli	Adds fixed_alloc_base to config interface.
lib/testGeneration/testGenConfig.ml	Stores fixed_alloc_base in config + accessor.
lib/testGeneration/buildScripts/make.ml	Emits --fixed-alloc-base into generated Make-based runners.
lib/testGeneration/buildScripts/bash.ml	Emits --fixed-alloc-base into generated Bash runners.
bin/test.ml	Adds --fixed-alloc-base CLI flag and threads it into TestGeneration config.

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