Fix testroms project compilation

fixes e8c957a7a

src/BizHawk.Client.Common/DisplayManager/DisplayManagerBase.cs

@@ -42,6 +42,8 @@ namespace BizHawk.Client.Common
 			}
 		}
 
+		public const int DEFAULT_DPI = 96;
+
 		public OSDManager OSD { get; }
 
 		protected Config GlobalConfig;

src/BizHawk.Client.EmuHawk/DisplayManager/DisplayManager.cs

@@ -52,7 +52,8 @@ namespace BizHawk.Client.EmuHawk
 
 		public override Size GetPanelNativeSize() => _presentationPanel.NativeSize;
 
-		protected override int GetGraphicsControlDpi() => OSTailoredCode.IsUnixHost ? 96 : GraphicsControlDpi;
+		protected override int GetGraphicsControlDpi()
+			=> OSTailoredCode.IsUnixHost ? DisplayManagerBase.DEFAULT_DPI : GraphicsControlDpi;
 
 		// DeviceDpi is not available under mono, so we need to have this access wrapped in a separate function
 		private int GraphicsControlDpi => _graphicsControl.DeviceDpi;

src/BizHawk.Tests.Testroms.GB/DummyFrontend.cs

@@ -98,6 +98,9 @@ namespace BizHawk.Tests.Testroms.GB
 
 			public override void ActivateOpenGLContext() {}
 
+			protected override int GetGraphicsControlDpi()
+				=> DisplayManagerBase.DEFAULT_DPI;
+
 			protected override Size GetGraphicsControlSize()
 				=> throw new NotImplementedException();