Added System.Diagnostics.Process tests.

Added AssemblyInfo.cs file to Microsoft.Win32.Primitives, upped the version number to maximum. Tests dependent on this assembly and looking for any version of this assembly should be able to pick this up.
Fix dirs.proj to pick up csproj files that don't immediately exist under test directory but a level below.
Shouldn't P/Invoke methods be moved to separate `NativeMethods`, `SafeNativeMethods` or `UnsafeNativeMethods` classes? This would allow sharing the methods across multiple test classes.