tests: Add tests for differentiation
tests: Add tests for differentiation

This ensures that the zero-th derivative returns the correct answer.

The sympy python library returns the 'correct' answer. There are still
some other deviations which are documented further in the next commit.

Signed-off-by: Garming Sam <95d1856ad7e7e0b74b122929c1f1e47d287c99a8@catalyst.net.nz>

bjodah/symengine.py,symengine/symengine.py,symengine/symengine.py,bjodah/symengine.py,symengine/symengine.py,bjodah/symengine.py