r.viewshed.exposure: Change nproc to 4 to speed up CI tests

[echoix]

I measured about a 30sec time reduction by not using 10 cores, while the runners have 4 cores available. However, the r.univar stats didn't match exactly for the sum of test_lakes, at least not with a precision of 1E-5. So I measured multiple points, with different nproc values, to see what would be a good choice.

I chose midpoint of 1 and 10 core (on x64), they were the min and max before the 20 core that was further off. With a 4E-5 precision, all of the nproc values would be included.

Here is a bit of the commented code before I removed it through the last commit. Let me know if it should be kept, or having the info in the PR is enough.

            "sum": 31834.1480130134,  # midpoint of 1 and 10 core x64
            # "sum": 31834.1480232505,  # average 1, 2, 4 and 10 core
            # "sum": 31834.1480175955,  # average 1, 4 and 10 core
            # "sum": 31834.1480048643,  # average 4 and 10 core
            # "sum": 31834.1480430579,  # 1 core x64
            # "sum": 31834.1480402155,  # 2 core x64
            # "sum": 31834.1480267597,  # 4 core x64
            # "sum": 31834.1480075223,  # 8 core x64
            # "sum": 31834.1479829689,  # 10 core x64
            # "sum": 31834.1479833564,  # 12 core x64
            # "sum": 31834.1479394501,  # 20 core x64