ONNX_MLIR_BUILD_TESTS doesn't appear to be honored

[mschofie]

The "ONNX_MLIR_BUILD_TESTS" setting is documented as controlling whether the test executables are built. But it doesn't appear to be honored. A couple of problems:

1. It's only tested in docs/doc_example/CMakeLists.txt and test/multiple-models/CMakeLists.txt
2. When it's ON, the logic sets set(EXCLUDE_FROM_ALL ON). The CMake documentation for EXCLUDE_FROM_ALL says that it's either a property on a target (documentation), or a property on a directory (documentation) - and set(EXCLUDE_FROM_ALL ON) sets neither.

It looks like a fix would be to:

1. Test it in docs/doc_example/CMakeLists.txt and test/CMakeLists.txt
2. Set the property on the directory with (for example):

# If 'ONNX_MLIR_BUILD_TESTS' is ON, set 'EXCLUDE_FROM_ALL' for this directory. This will allow test targets to be
# defined, but 'all' will not depend on them by default.
if (NOT ONNX_MLIR_BUILD_TESTS)
  set_directory_properties(PROPERTIES EXCLUDE_FROM_ALL ON)
endif()

This problem manifests for me with a Visual Studio Generator, and the fix fixes it. I'll double check with a Ninja Generator. I'm happy to send the fix; wondering if others see this problem...?