Decouple test suite from compiler internals for pure blackbox testing

[sbc100]

Most tests in test/test_core.py and test/test_browser.py test emcc as a black box via CLI invocations. However, parts of test/test_other.py, test/test_sanity.py, and the test harness (test/common.py) directly import and depend on Python compiler internals (tools/*).

If we want to rewrite the compiler (or components like the linker) in another language, or run the test suite against alternative implementations, we need to eliminate these internal dependencies and make the suite purely blackbox.

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Breakdown & Estimated Effort

Area	Current Coupling	Proposed Fix	Effort
1. Global State Mutation	Tests for -print-* flags in test_other.py mutate options.lto and settings.MEMORY64 to predict paths via cache.get_lib_dir().	Assert on CLI output paths directly without altering Python singletons.	Small (~0.5 day)
2. Binary Type Checks	test_other.py calls building.is_wasm(), is_wasm_dylib(), is_ar().	Inspect magic bytes (b'\0asm') or use llvm-readobj / llvm-ar.	Small (~1 day)
3. Direct Unit Tests	test_other.py / test_browser.py unit-test Python functions directly (binary_encode, wasm_sourcemap, response_file, closure_compiler).	Test via CLI flags or isolate into a dedicated Python unit test directory.	Small (~1–2 days)
4. Wasm Inspection	~15 tests in test_core.py and test_other.py use tools.webassembly.Module to check section names/features.	Replace with standard CLI tools (llvm-readobj, llvm-objdump -h, wasm-dis).	Medium (~2–3 days)
5. Cache & Ports Introspection	Tests query cache.get_sysroot_dir() or ports.Ports.get_dir() to find files in the cache.	Expose paths via CLI (e.g. emcc --print-sysroot) and test ports via --use-port.	Medium (~2 days)
6. Harness Settings Filtering	common.py imports COMPILE_TIME_SETTINGS to filter -s flags during compile-only (-c) runs.	Make compiler ignore/warn on link settings during -c, or query capabilities via CLI.	Medium (~2–3 days)
7. Sanity Suite Partitioning	test_sanity.py (~1,000 lines) tests the Python toolchain driver (config.py, cache locking, version checks).	Separate Python driver tests from the language-agnostic conformance test suite.	Medium (~1–2 days)

Total Estimated Effort: ~2 to 3 weeks

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Next Steps

1. Clean up quick wins in test_other.py (remove options/settings mutation, replace building.is_* helpers).
2. Replace tools.webassembly calls with standard binary tools (llvm-readobj).
3. Decouple harness dependencies (COMPILE_TIME_SETTINGS) and separate test_sanity.py.

[sbc100]

@dschuff hopefully this decoupling will make any emscripten-rewrite attempts easier. We can keep the test suite static while mutating the compiler as we like