Re-enable argument forwarding tests

[dsplaisted]

The argument forwarding tests are being disabled as part of dotnet/cli#7478, due to https://github.com/dotnet/corefx/issues/23496. We should re-enable them once that bug is fixed, or when we get a version of MSBuild with dotnet/msbuild#2467, which also works around the issue.

[livarcocc]

This seems to have been fixed on MSBuild.

[livarcocc]

These tests have been re-enabled a while ago.