* Reduce test target complexity [and running time]
* WiP
* Enable building tests via solution
Remove deprecated tests
Make Microsoft.DotNet.Tools.Tests.Utilities portable-only
Remove MSI tests from the solution as they are the only tests that currently require dekstop.
* Enable building of tests
* Move migration tests to TA to allow them to self-restore
* Reduce project nesting and test directory name
* Fixing argument parsing in add package command
* Adding check to throw if extra args were passed
* Removing string and adding test cases for dotnet add package command
* Add new test to test.sln, and fix naming, and clean csproj
This commit introduces a solution file for test projects and modifies
the needed build targets to restore the test solution instead of each
test project separately.
It also introduces a new template called "sln" to dotnet new that drops
a template solution file for users to consume.
Fixes#5099
