math module test portability (t440.py)

t440.py fails on my machine (Linux on ThinkPad) in trying to correctly compute the tangent of pi/2 and pi. This appears to be due to implementation details in math coprocessors rather than an error in the Skulpt Python math module wrapper. I tried to retain as many of the existing math.tan tests as possible by implementing a function isCloseTo(expected, actual, precision). The tan(pi/2) test was lost and the tan(pi) test was retained with this approximate test.