Optimize tests for surfaces

[felenitaribeiro]

When building tests to check whether surfaces were appropriately loaded, we noticed that it takes a little while to detect them. Currently, we have to load two surfaces to change the state of whether a surface was loaded and then be able to test for the loading by checking the output about the number of points/nodes as in:

${isMesh.value? getNumberOfPoints(nvArraySelected.value[0]): 'No mesh'}

in the niivue/src/components/Menu.ts file.

The same problem happens with the surface overlay.