Increase clarity in test case skipping

[ellahathaway]

In the Microsoft.DotNet.SourceBuild.SmokeTests.SdkContentTests, we include both Config.MsftSdkTarballPath and Config.SdkTarballPath as variables to check for skipping the test. That is, if either of these variables is null or contains whitespace, the test will be skipped. This approach lacks transparency from a user experience perspective, as it doesn't clearly indicate which variable triggered the test to skip.

A more user-friendly solution would involve introducing a variable such as "IncludeSdkContentTests" that determines whether or not to skip the test, and subsequently asserting that neither Config.MsftSdkTarballPath nor Config.SdkTarballPath is null or contains whitespace. By adopting this approach, we empower users to discern which specific variable caused the test to be omitted.

[ellahathaway]

This will be closed with the changes from dotnet/installer#19290