test: tweak bug273.go to not have constant len < cap when calling make

test: tweak bug273.go to not have constant len < cap when calling make

The current spec says that when calling make, if both len and
cap are constant, it is an error if len > cap.  The gc
compiler does not yet implement that, but when it does this
will need to change.  Changing it now for the benefit of
gccgo.

R=gri
CC=golang-dev
https://golang.org/cl/6867064