ProcessTests: redirect stdio and kill entire process tree for shell-script-based tests.

src/libraries/System.Diagnostics.Process/tests/ProcessTests.Unix.cs

@@ -173,7 +173,13 @@
             File.WriteAllText(filename, $"#!/bin/sh\nsleep 600\n"); // sleep 10 min.
             File.SetUnixFileMode(filename, ExecutablePermissions);
 
-            using (var process = Process.Start(new ProcessStartInfo { FileName = filename }))
+            using SafeFileHandle nullHandle = File.OpenNullHandle();
+            ProcessStartInfo psi = new(filename)
+            {
+                StandardOutputHandle = nullHandle,
+                StandardErrorHandle= nullHandle
+            };
+            using (var process = Process.Start(psi))
             {
                 try
                 {
@@ -186,7 +192,7 @@
                 }
                 finally
                 {
-                    process.Kill();
+                    process.Kill(entireProcessTree: true);
                     process.WaitForExit();
                 }
             }

src/libraries/System.Diagnostics.Process/tests/ProcessTests.cs

@@ -2499,7 +2499,13 @@ public void LongProcessNamesAreSupported()
             string sleepCommandPathFileName = Path.Combine(TestDirectory, LongProcessName);
             File.Copy(sleepPath, sleepCommandPathFileName);
 
-            using (Process px = Process.Start(sleepCommandPathFileName, "600"))
+            using SafeFileHandle nullHandle = File.OpenNullHandle();
+            ProcessStartInfo psi = new(sleepCommandPathFileName, "600")
+            {
+                StandardOutputHandle = nullHandle,
+                StandardErrorHandle= nullHandle
+            };
+            using (Process px = Process.Start(psi))
             {
                 // Reading of long process names is flaky during process startup and shutdown.
                 // Wait a bit to skip over the period where the ProcessName is not reliable.
@@ -2512,7 +2518,7 @@ public void LongProcessNamesAreSupported()
                 }
                 finally
                 {
-                    px.Kill();
+                    px.Kill(entireProcessTree: true);
                     px.WaitForExit();
                 }
             }