collate/build: do not use println in tests

Change-Id: I4f50430a35b009618c4762c74c418919dc54cdb1
Reviewed-on: https://go-review.googlesource.com/c/text/+/634495
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: alexandear

collate/build/contract_test.go

@@ -247,7 +247,6 @@ func TestPrintContractionTrieSet(t *testing.T) {
 	print(&testdata, buf, "test")
 	if contractTrieOutput != buf.String() {
 		t.Errorf("output differs; found\n%s", buf.String())
-		println(string(buf.Bytes()))
 	}
 }