zstd_test.go: Fix TestCompressLevelStackCgoBug to actually crash

This bug is very sensitive to stack sizes. Calling Decompress caused
that function to copy the stack, which then avoided the bug.

zstd_test.go

@@ -114,13 +114,8 @@ func doCompressLevel(payload []byte, out []byte) error {
 	if err != nil {
 		return fmt.Errorf("failed calling CompressLevel: %w", err)
 	}
-
-	orig, err := Decompress(nil, out)
-	if err != nil {
-		return fmt.Errorf("failed calling Decompress: %w", err)
-	}
-	if !bytes.Equal(orig, payload) {
-		return fmt.Errorf("orig=%#v should match payload=%#v", string(orig), string(payload))
+	if len(out) == 0 {
+		return errors.New("CompressLevel must return non-empty bytes")
 	}
 	return nil
 }