runtime: make TestGcSys actually test something

The workthegc function was being inlined, and the slice did not
escape, so there was no memory allocation. Use a sink variable to
force memory allocation, at least for now.

Fixes #23343

Change-Id: I02f4618e343c8b6cb552cb4e9f272e112785f7cf
Reviewed-on: https://go-review.googlesource.com/122576
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/testdata/testprog/gc.go

@@ -48,8 +48,11 @@ func GCSys() {
 	fmt.Printf("OK\n")
 }
 
+var sink []byte
+
 func workthegc() []byte {
-	return make([]byte, 1029)
+	sink = make([]byte, 1029)
+	return sink
 }
 
 func GCFairness() {