This is a pretty big refactoring, which:
* Always copies the test projects to a temporary directory before running any tests
that use them.
* Runs all the tests using an out-of-process MSBuild instance.
* Logs to a binlog instead of writing text to stdout.
* Refactors all the code that used the MSBuild assemblies in memory to instead:
* Tests that modified projects in memory now modifies them on disk instead. This
won't affect the working copy because the tests are always working with a copy
of the test projects.
* Tests that inspected projects in memory afterwards now parses the binlog to get
the same information.
* Significantly simplified the code to setup the test projects for testing.
Split the iOS msbuild tests in two:
* Xamarin.MacDev.Tasks.Tests: contains in-process unit tests for tasks.
* Xamarin.MacDev.Tasks: contains larger tests that either invoke targets or a complete
build. These are currently in-process, but will become out-of-process soon to make
it possible to run them with dotnet.
Also make the new projects non-iOS-specific, because the macOS msbuild tests will
be moved here as well soon.
There is some duplicated code between these two test projects now (all files
that show up as new are copies of existing files), this will be cleaned up in
later pull requests.