Add real functional tests for decimal places of numbers formatted by system format

[JasonYeMSFT]

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[craxal]

@JasonYeMSFT @MRayermannMSFT Can we narrow in on the scope of tests being asked for? This is not trivial. Open to ideas, but the trick will be being able to predict the locale and format settings of the test machine. I would think we would want to use different settings to test other formats as well, which makes writing these even harder.

[JasonYeMSFT]

I updated the title. We are interested in testing the number of digits after the decimal points if we explicitly specify how many digits to use, which doesn't depend on the local setting of the build agent.