Flaky tests

[germa89]

To fix one day...

• [x] test_heal #2173

  [FAILED] test_heal - E       AttributeError: 'NoneType' object has no attribute 'exit'
  

• [ ] test_empty_mesh. From: https://github.com/ansys/pymapdl/pull/2443/commits/1c5b3a1868cc73636a0d0dca595bb9677d066381

  [FAILED] test_empty_mesh - E           ansys.mapdl.core.errors.MapdlConnectionError: The channel is not alive.
  

• [ ] test_plot_incomplete_nodal_selection. Regression image fail. Related to https://github.com/ansys/pymapdl/issues/2301

  From https://github.com/ansys/pymapdl/pull/2443/commits/f535d6e90f372ec98cbcabeab26f6aabd7f13069

[germa89]

Adding because of #3266 :

• [ ] test_launch_mapdl_cli_list. It does not seems to detect the running MAPDL.

   AssertionError: assert ('running' in 'Name    Is Instance    Status    gRPC port    PID\n------  -------------  --------  -----------  -----\n' or 'sleeping' in 'Name    Is Instance    Status    gRPC port    PID\n------  -------------  --------  -----------  -----\n')
  

• [ ] test_map_timeout. Probably there is a lag in some of the threads which makes one of the instances not being killed on time.
• [ ] test__remove_temp_dir_on_exit. Because there is another MAPDL instance running, probably the test before this one (test__remove_temp_dir_on_exit) left the instance behind.
• [ ] test_remove_temp_dir_on_exit. Same as before.
• [ ] Additionally, I know some instances are left alive after each testing. I should check/activate run_before_and_after_tests_3 fixture.

[germa89]

Test heal is not flaky anymore since https://github.com/ansys/pymapdl/pull/2493