BUG: off-by-one when doing network test repeats

Imagine num_runs is two, then runs will take on the values [0, 1] and runs <
num_uns will always be False. Hence any genuine Exception the code might raise
will be silently ignored and just printed.
BUG/TST: fix (and skip) tests that have been failing unnoticed all along

The test is unstable and although this makes it work properly, no one knows how
long. Hence it was decided to skip this guy until further notice.

See also: #4427

Skip test_fred_nan and test_fred_parts, both of which have been failing silently for a while