Fix target platform of managed OnnxRuntime dll and enable x86 .NET testing (#3056)

* WIP: Re-enable x86 .NET testing in Release pipelines

Enabling x86 testing will make sure that ORT packages doesn’t break x86 projects of customers

* Remove setting some env variables

* Comment out a test failing on x86 builds

* More changes

* Minor fix

* More changes

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* Revert minor change

* More changes

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* More changes 2

* explicitly set platform target

* Delete bin and obj folders

* Clean output dirs

* Add back TargetFramwork

* Disable x86 .net framework tests

* Skip x86 tests in MKLML pipeline