Fix spawn tests in different environments

We are running our tests in different environments, sometimes with a
terminal as stdin and sometimes with /dev/null as stdin.  Both are seen
as "not a pipe" by `test -p` so we use that to test if our python code
redirects text into an external process.

flake.nix

@@ -36,10 +36,6 @@
 
             nativeCheckInputs = with pkgs; [ gnupg notmuch procps ];
             checkPhase = ''
-              # In the nix sandbox stdin is not a terminal but /dev/null so we
-              # change the shell command only in this specific test.
-              sed -i '/test_no_spawn_no_stdin_attached/,/^$/s/test -t 0/sh -c "[ $(wc -l) -eq 0 ]"/' tests/commands/test_global.py
-
               python3 -m unittest -v
             '';
           });

tests/commands/test_global.py

@@ -124,19 +124,19 @@ async def test_no_spawn_stdin_success(self):
     @utilities.async_test
     async def test_no_spawn_no_stdin_attached(self):
         ui = utilities.make_ui()
-        cmd = g_commands.ExternalCommand('test -t 0', refocus=False)
+        cmd = g_commands.ExternalCommand('test -p /dev/stdin', refocus=False)
         await cmd.apply(ui)
-        ui.notify.assert_not_called()
+        ui.notify.assert_called_once_with(
+                'editor has exited with error code 1 -- No stderr output',
+                priority='error')
 
     @utilities.async_test
     async def test_no_spawn_stdin_attached(self):
         ui = utilities.make_ui()
         cmd = g_commands.ExternalCommand(
-            "test -t 0", stdin='0', refocus=False)
+            "test -p /dev/stdin", stdin='0', refocus=False)
         await cmd.apply(ui)
-        ui.notify.assert_called_once_with(
-                'editor has exited with error code 1 -- No stderr output',
-                priority='error')
+        ui.notify.assert_not_called()
 
     @utilities.async_test
     async def test_no_spawn_failure(self):