Visual Studio 10 uses MSBuild to drive the build.  Custom commands
appear in MSBuild files inside CustomBuild elements, which appear inside
ItemGroup elements.  The Outputs and AdditionalInputs elements of each
CustomBuild element are evaluated according to timestamps on disk.

MSBuild does not use inputs/outputs to order CustomBuild steps within a
single ItemGroup or across multiple ItemGroup elements.  Instead we must
put only unrelated CustomBuild elements in a single ItemGroup and order
the item groups from top to bottom using a topological order of the
custom command dependency graph.

This fixes CustomCommand and ExternalProject test failures, so we remove
the expectation of these failures.