Enable tests on Windows

[tlemo]

Currently, the scaffolding used to run the tests is not working on Windows

[tlemo]

I think I have most of the changes required to enable tests on Windows, but there're an unfortunate bump - an amusing MSVC++ bug:

tf_runtime\external\llvm-project\mlir\include\mlir/IR/Location.h(120): error C2440: 'reinterpret_cast': cannot convert from 'T' to 'uintptr_t'
        with
        [
            T=uintptr_t
        ]
tf_runtime\external\llvm-project\mlir\include\mlir/IR/Location.h(120): note: Conversion is a valid standard conversion, which can be performed implicitly or by use of static_cast, C-style cast or function-style cast
cpp_tests/bef_converter/bef_location_emitter_test.cc(56): note: see reference to function template instantiation 'mlir::Location mlir::OpaqueLoc::get<uintptr_t>(T,mlir::MLIRContext *)' being compiled
        with
        [
            T=uintptr_t
        ]

https://developercommunity.visualstudio.com/t/Bogus-C2440-involving-reinterpret_cast-w/1480040