CLI: verdi computer test make unexpected output check optional (#6384)

The test for unexpected output in `verdi computer test` requires to
execute a command on the remote, however, this may not be implemented
by all transport plugins. If `NotImplementedError` is raised, this now
no longer fails the test but prints why the test was skipped.

src/aiida/cmdline/commands/cmd_computer.py

@@ -62,14 +62,24 @@ def _computer_test_no_unexpected_output(transport, scheduler, authinfo, computer
     This can happen if e.g. there is some spurious command in the
     .bashrc or .bash_profile that is not guarded in case of non-interactive
     shells.
+    Note: This test is irrelevant if the transport plugin does not support command execution.
 
     :param transport: an open transport
     :param scheduler: the corresponding scheduler class
     :param authinfo: the AuthInfo object (from which one can get computer and aiidauser)
     :return: tuple of boolean indicating success or failure and an optional string message
     """
-    # Execute a command that should not return any error
-    retval, stdout, stderr = transport.exec_command_wait('echo -n')
+    # Execute a command that should not return any error, except ``NotImplementedError``
+    # since not all transport plugins implement remote command execution.
+    try:
+        retval, stdout, stderr = transport.exec_command_wait('echo -n')
+    except NotImplementedError:
+        return (
+            True,
+            f'Skipped, remote command execution is not implemented for the '
+            f'`{computer.transport_type}` transport plugin',
+        )
+
     if retval != 0:
         return False, f'The command `echo -n` returned a non-zero return code ({retval})'