Run 'board list' test anyway

Even if no board are attached to the host system, it's always another
smoke-test that runs automatically.

Having this check enabled, for example, would have prevented:
arduino#904

Author: cmaglie

test/test_board.py

@@ -372,7 +372,6 @@
 """  # noqa: E501
 
 
-@pytest.mark.skipif(running_on_ci(), reason="VMs have no serial ports")
 def test_board_list(run_command):
     result = run_command("core update-index")
     assert result.ok