fix testing of mip solvers
fix testing of mip solvers

aebrahim/cobrapy,jeicher/cobrapy,jerkos/cobrapy,jerkos/cobrapy,aebrahim/cobrapy,jeicher/cobrapy,JuBra/cobrapy,zakandrewking/cobrapy,zakandrewking/cobrapy,JuBra/cobrapy