test: avoid "declared but not used" errors in shift1.go

I'm improving gccgo's detection of variables that are only set
but not used, and it triggers additional errors on this code.
The new gccgo errors are correct; gc seems to suppress them
due to the other, expected, errors.  This change uses the
variables so that no compiler will complain.

gccgo change is https://golang.org/cl/119920043 .

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/116050043

test/shift1.go

@@ -238,4 +238,6 @@ func _() {
 	z = (1. << s) << (1 << s)    // ERROR "non-integer|type complex128"
 	z = (1. << s) << (1. << s)   // ERROR "non-integer|type complex128"
 	z = (1.1 << s) << (1.1 << s) // ERROR "invalid|truncated|complex128"
+
+	_, _, _ = x, y, z
 }