Add extension/llm/custom_ops/spinquant/test to CMake

[swolchok]

Need to run it to make sure build changes in this directory aren't breaking things.

Test Plan: added EXPECT_TRUE(false) to fast_hadamard_transform_test.cpp, saw failure in test/run_oss_cpp_tests.sh

[swolchok]

Stack from ghstack (oldest at bottom):

• build_variables.bzl: split PLATFORM_SRCS from EXECUTORCH_CORE_SRCS #8398
• build_variables.bzl: split PROGRAM_NO_PRIM_OPS_SRCS from EXECUTORCH_CORE_SRCS #8397
• create and validate build_variables.bzl #8326
• determine fht_avx.c/fht_neon.c based on CMake target processor #8346
• ->Add extension/llm/custom_ops/spinquant/test to CMake #8345
• Bypass find_package(torch) for find_package_torch_headers to help out cross compilation #8629

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8345

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✅ You can merge normally! (1 Unrelated Failure)

As of commit b52f223 with merge base f3fc096 ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llava-runner-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[swolchok]

this isn't correct yet; test doesn't run when I run test/run_oss_cpp_tests.sh and I'm not sure why yet.

[swolchok]

temporarily adding --trace to the problem cmake command in build-llm-demo since I can't repro locally

[swolchok]

temporarily adding --trace to the problem cmake command in build-llm-demo since I can't repro locally

the correct cmake command was in extension/android_test/setup.sh, but I still can't figure out what's going on so now i'm just tracing all 4 cmake invocations

[swolchok]

the inexplicable build of tests seems to be because including abseil-cpp's CMakeLists.txt sets BUILD_TESTING to ON by default because it does include(CTest). trying out find_package(GTest) in et_cxx_test to see if it fixes the issue

[swolchok]

this looks good, please review

[swolchok]

ping reviewers @kirklandsign@kimishpatel@larryliu0820@jackzhxng@iseeyuan@GregoryComer@dbort