Disable SMP tools during testing

[banesullivan]

With the addition of enable_smp_tools in pyvista, we should add a fixture like this to disable it automatically.

# conftest.py                                                                                                                                  
import pytest                                                                                                                                  
import pyvista as pv                      
                                                                                                                                               
@pytest.fixture(scope='session', autouse=True)                     
def _disable_smp_in_tests():              
    pv.enable_smp_tools(backend='sequential', n_threads=1)