Improve test coverage in `Quantity.to_string()`
Improve test coverage in `Quantity.to_string()`

StuartLittlefair/astropy,mhvk/astropy,mhvk/astropy,astropy/astropy,StuartLittlefair/astropy,pllim/astropy,StuartLittlefair/astropy,lpsinger/astropy,pllim/astropy,astropy/astropy,mhvk/astropy,saimn/astropy,lpsinger/astropy,mhvk/astropy,mhvk/astropy,lpsinger/astropy,larrybradley/astropy,larrybradley/astropy,lpsinger/astropy,pllim/astropy,astropy/astropy,pllim/astropy,saimn/astropy,saimn/astropy,astropy/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,larrybradley/astropy,pllim/astropy,saimn/astropy,larrybradley/astropy,larrybradley/astropy,StuartLittlefair/astropy,lpsinger/astropy