test: recognize gccgo error message in index.go

When a floating point constant is used as an array/slice
index, gccgo prints "error: index must be integer"; gc prints
"constant 2.1 truncated to integer".

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/14044044

test/index.go

@@ -164,7 +164,7 @@ func testExpr(b *bufio.Writer, expr string) {
 	if pass == 0 {
 		fmt.Fprintf(b, "\ttest(func(){use(%s)}, %q)\n", expr, expr)
 	} else {
-		fmt.Fprintf(b, "\tuse(%s)  // ERROR \"index|overflow|truncated\"\n", expr)
+		fmt.Fprintf(b, "\tuse(%s)  // ERROR \"index|overflow|truncated|must be integer\"\n", expr)
 	}
 }