Allow macOS processes to return 128 on kill

The exit code of processes killed using our timeout appears to have
changed on macOS after taking .NET Core 2.1 preview 2. I have no idea
why, but it's consistent on my laptop and in CI. Adjusting the test
to expect 128 on Mac.

src/Tasks.UnitTests/Exec_Tests.cs

@@ -93,8 +93,12 @@ public void ExitCodeCausesFailure()
         [Fact]
         public void Timeout()
         {
-            // On non-Windows the exit code of a killed process is SIGTERM (143)
-            int expectedExitCode = NativeMethodsShared.IsWindows ? -1 : 143;
+            // On non-Windows the exit code of a killed sh should be
+            // SIGTERM (15) + exited-due-to-signal (128) = 143.
+            // On .NET Core 2.1 preview2 + High Sierra it's consistently
+            // just 128 for some reason.
+            int expectedExitCode = NativeMethodsShared.IsWindows ? -1 :
+                    NativeMethodsShared.IsOSX ? 128 : 143;
 
             Exec exec = PrepareExec(NativeMethodsShared.IsWindows ? ":foo \n goto foo" : "while true; do sleep 1; done");
             exec.Timeout = 5;