Downgrade Microsoft.NET.Test.Sdk to net40 and fix casing of msbuild files

[ViktorHofer]

Fixes #1850
Fixes #1863

There is still the dependency to Microsoft.CodeCoverage with net45 but a downgrade isn't possible here as Microsoft.CodeCoverage builds on netstandard1.0 which isn't supported with net40.

<group targetFramework="net45">
        <!-- TestHost gets shipped with vstest.console -->
        <dependency id="Microsoft.CodeCoverage" version="$Version$" />
      </group>
    </dependencies>

[ViktorHofer]

cc @singhsarab @mayankbansal018 @jnm2

[singhsarab]

@ViktorHofer Have you validated that the props is getting honored for a net40 project ?

[ViktorHofer]

Yes just verified that the referencing the package from a net40 app works fine and the props and targets files are honored.

[singhsarab]

@ViktorHofer Thanks for the contribution 👍

[ViktorHofer]

You are welcome. Please let me know if/when you push a new package.