Test by expand_mul instead of random rationals
Test by expand_mul instead of random rationals

Might be slightly slower - 88s vs 84 seconds in my test of 10 M.det()
calculations with M being
Matrix([[i + a*j + S(1)/(i+j+1) for i in range(7)] for j in range(7)])
But this is likely to have fewer incorrect results than the previous
testing method.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy