Resolve WindowsDesktop and AspNetCore tests correctly

[CharliePoole]

Fixes #1274

As noted, this fix backs out the changes, which resulted in issue #1274 and adds code to the resolver to directly locate the two additional frameworks, Microsoft.WindowsDesktop.App and Microsoft.AspNetCore.App, when the test assembly depends on one of them.

This is a major change and should have a code review if someone has the time. I'll leave it for a few days before merging.

[mikkelbu]

I'll take a look now

[mikkelbu]

I don't know much about this area of the codebase, but as far as I can tell the changes look sound. My main worry is whether the regkey always can be used (in windows) and whether the path is always the correct one in Linux (or we should look it up - using e.g. which dotnet or whereis dotnet).

[CharliePoole]

@mikkelbu Thanks for reviewing. A general answer to your general comment... I'm too suspect that non-standard installs could break this, particularly on Linux. However it wouldn't be a new issue - the same key and linux path is what we already use to figure out what versions of the runtime are installed.

In general, I think non-standard install locations are intended to be hidden from general use. This is similar to when you install a tool to a special path... dotnet cli can't find the tool unless you tell it the path in some way.