build: exclude mptest target from compile commands #35418

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.

This PR excludes the mptest target from the compilation database by setting EXPORT_COMPILE_COMMANDS OFF, following the same pattern already applied to the mpcalculator, mpprinter, and mpexample targets in the same file.

This change belongs in cmake/libmultiprocess.cmake (the integration layer).

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35418.

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