wire: fix test data race (google/go-cloud#168)

This was introduced in 7d83fb8532c1455d4e81b587f31d090320f5eb9f and is
surprisingly not caught by the race detector. I suspect that the testing
package makes it not a strict data race, but the result is still
undefined.

internal/wire/wire_test.go

@@ -67,6 +67,7 @@ func TestWire(t *testing.T) {
 			t.Skip("go toolchain not available:", err)
 		}
 		for _, test := range tests {
+			test := test
 			t.Run(test.name, func(t *testing.T) {
 				t.Parallel()
 				bctx := test.buildContext()
@@ -137,6 +138,7 @@ func TestWire(t *testing.T) {
 			if test.wantError {
 				continue
 			}
+			test := test
 			t.Run(test.name, func(t *testing.T) {
 				t.Parallel()
 				bctx := test.buildContext()