build: exclude libmultiprocess test target from compile commands #35401

Fixes #35361

The mptest target includes generated .capnp.h files that don't exist at CMake configure time. When IWYU reads compile_commands.json, it sees these non-existent files and fails with errors like: fatal error: 'mp/test/foo.capnp.h' file not found This PR excludes the mptest target from the compilation database by setting EXPORT_COMPILE_COMMANDS OFF, following the same pattern already applied to the libmultiprocess example targets in PR #31741.

Before this change:

• IWYU would fail on libmultiprocess test files
• CI logs show sporadic "file not found" errors

After this change:

• The test target is excluded from compile_commands.json
• IWYU no longer attempts to check these files
• Cleaner CI logs with no false positive errors

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35401.

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/26562892028/job/78250708763} _{LLM reason (✨ experimental): CI failed the lint “subtree” check because a subtree directory was modified without a corresponding subtree merge.}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Thx, but according to the CI failure, this is the wrong repo. The subtree will have to be fixed upstream.