[link sdk] Update test to work on macOS as well.

tests/linker/ios/link sdk/LinkSdkRegressionTest.cs

@@ -830,8 +830,13 @@ namespace LinkSdk {
 		{
 			// ref: https://bugzilla.xamarin.com/show_bug.cgi?id=21882
 #if NET
+#if __MACOS__
+			var mem = System.Diagnostics.Process.GetCurrentProcess ().PrivateMemorySize64;
+			Assert.That (mem, Is.EqualTo (0), "PrivateMemorySize64");
+#else
 			// It's not entirely clear, but it appears this is not implemented, and won't be, for mobile platforms: https://github.com/dotnet/runtime/issues/28990
 			Assert.Throws<PlatformNotSupportedException> (() => { var mem = System.Diagnostics.Process.GetCurrentProcess ().PrivateMemorySize64; }, "PrivateMemorySize64");
+#endif // __MACOS__
 #else
 			var mem = System.Diagnostics.Process.GetCurrentProcess ().PrivateMemorySize64;
 			// the above used a mach call that iOS samdbox did *not* allow (sandbox) on device