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

test/fixedbugs/bug273.go

@@ -11,6 +11,7 @@ package main
 var bug = false
 
 var minus1 = -1
+var five = 5
 var big int64 = 10 | 1<<32
 
 type block [1<<19]byte
@@ -40,7 +41,7 @@ func badcap() {
 }
 
 func badcap1() {
-	g1 = make([]block, 10, 5)
+	g1 = make([]block, 10, five)
 }
 
 func bigcap() {