add tests to check new sum simplification properites
add tests to check new sum simplification properites

yashsharan/sympy,chaffra/sympy,yashsharan/sympy,madan96/sympy,aktech/sympy,aktech/sympy,souravsingh/sympy,madan96/sympy,aktech/sympy,kaushik94/sympy,skidzo/sympy,souravsingh/sympy,jaimahajan1997/sympy,yashsharan/sympy,skidzo/sympy,chaffra/sympy,souravsingh/sympy,jaimahajan1997/sympy,kaushik94/sympy,kaushik94/sympy,madan96/sympy,chaffra/sympy,skidzo/sympy,jaimahajan1997/sympy