Run CTest tests serially to avoid oversubscribing cores

[horenmar]

The lit tests are already internally parallelized, so using CTest to run them in parallel ends up with the CPU cores hopelessly oversubscribed.

[fsb4000]

I've checked that the CI time doesn't change after the update, and I also consulted the CMake documentation to ensure there were no misspellings:
https://cmake.org/cmake/help/latest/command/set_tests_properties.html
https://cmake.org/cmake/help/latest/prop_test/RUN_SERIAL.html

[StephanTLavavej]

Thanks! I never run ctest (instead I always invoke stl-lit.py directly), so I didn't realize it was trying to run stl and stlasan simultaneously with separate Python invocations. Great catch 😻

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks again! 🎉 😸 🚀