Move `ScalarValue` tests alongside implementation, move `from_slice` to `core`

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I am working on #2891 and it was not clear there were existing ScalarValue tests because they were in a different module than their implementation

Implementation: https://github.com/apache/arrow-datafusion/blob/fd64e6f2e5ca5ba8c33c431b75a3ba9441091970/datafusion/common/src/scalar.rs
Tests https://github.com/apache/arrow-datafusion/blob/85ca8be669dc1fbba4b2eefbf99fc2ed1546cc0c/datafusion/core/src/scalar.rs

🤯
Describe the solution you'd like
Tests and implementation to be in the same module (per the usual rust convention

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

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