Add test file for #4957.
Add test file for #4957.

Bug #4957 was "unify cannot directly unify universes with evars, but can
do so indirectly".

gares/coq,ejgallego/coq,JasonGross/coq,amintimany/coq,SkySkimmer/coq,letouzey/coq-wip,Matafou/coq,ppedrot/coq,Zimmi48/coq,coq/coq,letouzey/coq-wip,ppedrot/coq,william-lawvere/coq,JasonGross/coq,maximedenes/coq,SkySkimmer/coq,coq/coq,silene/coq,letouzey/coq-wip,maximedenes/coq,silene/coq,maximedenes/coq,Matafou/coq,gares/coq,amintimany/coq,gares/coq,gares/coq,Zimmi48/coq,ejgallego/coq,SkySkimmer/coq,Zimmi48/coq,letouzey/coq-wip,coq/coq,Lionel-Rieg/coq,william-lawvere/coq,JasonGross/coq,silene/coq,maximedenes/coq,coq/coq,ppedrot/coq,ppedrot/coq,SkySkimmer/coq,maximedenes/coq,JasonGross/coq,amintimany/coq,Lionel-Rieg/coq,Matafou/coq,ppedrot/coq,Zimmi48/coq,ejgallego/coq,gares/coq,william-lawvere/coq,ejgallego/coq,letouzey/coq-wip,Lionel-Rieg/coq