Add reflection roots for tests calling CreateInstance() (dotnet/coreclr#10242)

Add reflection roots for tests calling CreateInstance() (dotnet/coreclr#10242)

Add the XML files identifying the reflection roots to keep the
type (and constructors threof) which are used by tests as
Activator.CreateInstance(typeof(T))

Commit migrated from https://github.com/dotnet/coreclr/commit/d1bdbf1bee3ecdbcd96a5a78a45bf097e3a38951