Add tests for issue 2736.
Add tests for issue 2736.

Before these commits ``ode_order`` incorrectly calculated order for expressions
like ``Derivative(x*f(x), x)``, because it assumed that any derivative is a
derivative of f(x) only, which is ussually not true (in the former example, we
have a derivative of a product, ``x*f(x)``).

garvitr/sympy,Gadal/sympy,oliverlee/sympy,yashsharan/sympy,Shaswat27/sympy,pbrady/sympy,ga7g08/sympy,Arafatk/sympy,ChristinaZografou/sympy,kaichogami/sympy,sampadsaha5/sympy,Curious72/sympy,kaichogami/sympy,ChristinaZografou/sympy,moble/sympy,abhiii5459/sympy,Titan-C/sympy,sahilshekhawat/sympy,drufat/sympy,meghana1995/sympy,postvakje/sympy,mcdaniel67/sympy,farhaanbukhsh/sympy,Davidjohnwilson/sympy,debugger22/sympy,liangjiaxing/sympy,farhaanbukhsh/sympy,emon10005/sympy,farhaanbukhsh/sympy,aktech/sympy,emon10005/sympy,kumarkrishna/sympy,saurabhjn76/sympy,grevutiu-gabriel/sympy,aktech/sympy,AunShiLord/sympy,souravsingh/sympy,skidzo/sympy,Gadal/sympy,atreyv/sympy,amitjamadagni/sympy,skirpichev/omg,chaffra/sympy,postvakje/sympy,diofant/diofant,jaimahajan1997/sympy,meghana1995/sympy,bukzor/sympy,debugger22/sympy,sahilshekhawat/sympy,mafiya69/sympy,bukzor/sympy,grevutiu-gabriel/sympy,debugger22/sympy,maniteja123/sympy,iamutkarshtiwari/sympy,hrashk/sympy,yukoba/sympy,jamesblunt/sympy,vipulroxx/sympy,souravsingh/sympy,postvakje/sympy,pbrady/sympy,toolforger/sympy,MridulS/sympy,MechCoder/sympy,flacjacket/sympy,drufat/sympy,abhiii5459/sympy,jaimahajan1997/sympy,jbbskinny/sympy,cccfran/sympy,asm666/sympy,iamutkarshtiwari/sympy,MechCoder/sympy,Designist/sympy,AkademieOlympia/sympy,sunny94/temp,cswiercz/sympy,toolforger/sympy,jamesblunt/sympy,bukzor/sympy,pandeyadarsh/sympy,sahmed95/sympy,kaushik94/sympy,beni55/sympy,madan96/sympy,yashsharan/sympy,kmacinnis/sympy,cswiercz/sympy,ga7g08/sympy,Curious72/sympy,amitjamadagni/sympy,pandeyadarsh/sympy,shikil/sympy,yukoba/sympy,jamesblunt/sympy,dqnykamp/sympy,cccfran/sympy,Mitchkoens/sympy,hargup/sympy,lidavidm/sympy,abloomston/sympy,VaibhavAgarwalVA/sympy,pandeyadarsh/sympy,MechCoder/sympy,sahmed95/sympy,Davidjohnwilson/sympy,oliverlee/sympy,kaushik94/sympy,sampadsaha5/sympy,AkademieOlympia/sympy,souravsingh/sympy,AkademieOlympia/sympy,lidavidm/sympy,skidzo/sympy,VaibhavAgarwalVA/sympy,Vishluck/sympy,mafiya69/sympy,Sumith1896/sympy,beni55/sympy,AunShiLord/sympy,Sumith1896/sympy,garvitr/sympy,jbbskinny/sympy,Curious72/sympy,kmacinnis/sympy,madan96/sympy,grevutiu-gabriel/sympy,Mitchkoens/sympy,shipci/sympy,Titan-C/sympy,Vishluck/sympy,toolforger/sympy,maniteja123/sympy,emon10005/sympy,atsao72/sympy,moble/sympy,mcdaniel67/sympy,kevalds51/sympy,kevalds51/sympy,iamutkarshtiwari/sympy,hargup/sympy,mafiya69/sympy,rahuldan/sympy,shikil/sympy,MridulS/sympy,kaichogami/sympy,jbbskinny/sympy,cswiercz/sympy,abhiii5459/sympy,VaibhavAgarwalVA/sympy,atsao72/sympy,maniteja123/sympy,mcdaniel67/sympy,shipci/sympy,kaushik94/sympy,rahuldan/sympy,jaimahajan1997/sympy,moble/sympy,atreyv/sympy,asm666/sympy,wyom/sympy,sunny94/temp,Shaswat27/sympy,ahhda/sympy,kevalds51/sympy,liangjiaxing/sympy,oliverlee/sympy,yashsharan/sympy,Arafatk/sympy,pbrady/sympy,Gadal/sympy,kumarkrishna/sympy,wanglongqi/sympy,chaffra/sympy,lindsayad/sympy,sahmed95/sympy,lindsayad/sympy,sampadsaha5/sympy,Designist/sympy,yukoba/sympy,liangjiaxing/sympy,hrashk/sympy,sahilshekhawat/sympy,asm666/sympy,saurabhjn76/sympy,Titan-C/sympy,kmacinnis/sympy,atreyv/sympy,lidavidm/sympy,jerli/sympy,hargup/sympy,ga7g08/sympy,madan96/sympy,meghana1995/sympy,MridulS/sympy,vipulroxx/sympy,Vishluck/sympy,vipulroxx/sympy,skidzo/sympy,ahhda/sympy,wanglongqi/sympy,cccfran/sympy,abloomston/sympy,kumarkrishna/sympy,abloomston/sympy,shipci/sympy,Davidjohnwilson/sympy,wanglongqi/sympy,jerli/sympy,drufat/sympy,srjoglekar246/sympy,dqnykamp/sympy,lindsayad/sympy,chaffra/sympy,Arafatk/sympy,AunShiLord/sympy,Mitchkoens/sympy,shikil/sympy,beni55/sympy,Designist/sympy,jerli/sympy,dqnykamp/sympy,ahhda/sympy,saurabhjn76/sympy,hrashk/sympy,sunny94/temp,rahuldan/sympy,Shaswat27/sympy,Sumith1896/sympy,ChristinaZografou/sympy,wyom/sympy,garvitr/sympy,aktech/sympy,wyom/sympy,atsao72/sympy