jumoo-context: local package installs on test sites

New-TestSite -LocalPackages <folder>: writes the site a NuGet.Config with package source mapping, so packages in the folder come from it and everything else from nuget.org, and pins a local package to its packed version
/full-test uses it for local builds. Fixes restores that floated to the next major (NU1107) because the case-sensitive Testing folder ignored a lowercase nuget.config