Do not bother with testing or setting POWERLINE_CONFIG_COMMAND
Do not bother with testing or setting POWERLINE_CONFIG_COMMAND

Instead just test whether using $POWERLINE_CONFIG_COMMAND succeeds and if it 
does not run just `powerline-config tmux setup`. Problem is that tests in vterm 
fail *not* because sleeping time is too short, but because `powerline-config 
tmux setup` for some reason is not launched at all. I am not sure whether this 
construct is going to fix things.

Also ref #875. I am very unsure this commit will fix the issue, but since I do 
not know what is the reason for this issue at all… who knows.

s0undt3ch/powerline,QuLogic/powerline,areteix/powerline,junix/powerline,russellb/powerline,s0undt3ch/powerline,S0lll0s/powerline,russellb/powerline,seanfisk/powerline,xxxhycl2010/powerline,Luffin/powerline,russellb/powerline,seanfisk/powerline,Luffin/powerline,blindFS/powerline,bezhermoso/powerline,EricSB/powerline,prvnkumar/powerline,QuLogic/powerline,seanfisk/powerline,prvnkumar/powerline,dragon788/powerline,bartvm/powerline,IvanAli/powerline,kenrachynski/powerline,kenrachynski/powerline,junix/powerline,Liangjianghao/powerline,s0undt3ch/powerline,areteix/powerline,DoctorJellyface/powerline,EricSB/powerline,dragon788/powerline,xfumihiro/powerline,bartvm/powerline,IvanAli/powerline,xxxhycl2010/powerline,bezhermoso/powerline,bezhermoso/powerline,junix/powerline,Luffin/powerline,Liangjianghao/powerline,cyrixhero/powerline,cyrixhero/powerline,EricSB/powerline,IvanAli/powerline,darac/powerline,kenrachynski/powerline,prvnkumar/powerline,dragon788/powerline,lukw00/powerline,S0lll0s/powerline,blindFS/powerline,areteix/powerline,lukw00/powerline,darac/powerline,Liangjianghao/powerline,xfumihiro/powerline,blindFS/powerline,DoctorJellyface/powerline,lukw00/powerline,xfumihiro/powerline,bartvm/powerline,cyrixhero/powerline,QuLogic/powerline,DoctorJellyface/powerline,darac/powerline,S0lll0s/powerline,xxxhycl2010/powerline