Added XFAIL test for dsolve

This is a dummy PR just to check if there is a bug in dsolve or not. In my master branch doing this.

    eq = x*(f(x).diff(x)) + 1 - f(x)**2
    dsolve(eq)
    f(x) == -1

I just wanted to know if there is something wrong with my master, or there actually is a bug.