tests : link each test against only the libraries it uses (#30299)
Give llama_build() and llama_build_and_test() a LINK keyword (default:
llama-common) so call sites can declare what a test actually needs:
- ggml-only tests now link just ggml: test-opt, test-metal-graph-optimize,
test-backend-ops, test-tiled-mulmat, test-barrier, test-quantize-fns,
test-quantize-perf, test-rope, test-col2im-1d, test-gguf, test-alloc
- llama-only tests drop llama-common: test-unicode, test-sampling,
test-grammar-parser, test-llama-grammar, test-batch-alloc,
test-gbnf-validator, test-quant-type-selection
- both mtmd tests link mtmd only, dropping the LLAMA_TEST_NAME /
target_link_libraries workarounds
Both helpers now set cxx_std_17 explicitly, since the tests no longer
inherit it from llama-common's PUBLIC interface.
tests/testing.h no longer includes common.h; the only symbol it used from
there (string_join) is inlined.
Assisted-by: pi:llama.cpp/Qwen3.8-Flash-Next