tests: add `-b/--backend` option to test-llama-archs for testing a specific backend (#27372)
* tests: add backend option to test-llama-archs
* Update tests/test-llama-archs.cpp
Co-authored-by: Johannes Gäßler <johannesg@5d6.de>
* remove extra space
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Co-authored-by: Johannes Gäßler <johannesg@5d6.de>