[Managed.Windows.Forms] Fixed System.Resources tests that relied on DummyAssembly

Several System.Resources tests rely on a DummyAssembly.dll, which was removed in 016b0b5170fca72b977f952ee817fe06e32890a9 (possibly by accident?).
This commit adds the assembly back.

Commit licensed under MIT/X11.
Merge pull request #1106 from akoeplinger/fix-winforms-tests

[Managed.Windows.Forms] Fixed System.Resources tests that relied on DummyAssembly