go/zodb: tests: Teach tDB.Commit to return committed TID

And use that in the callers.

go/zodb/persistent_test.go

@@ -317,7 +317,7 @@ func (t *tDB) Add(oid Oid, value string) {
 }
 
 // Commit commits objects queued by Add.
-func (t *tDB) Commit() {
+func (t *tDB) Commit() Tid {
 	t.Helper()
 
 	head, err := ZPyCommit(t.zurl, t.head, t.commitq...)
@@ -326,6 +326,7 @@ func (t *tDB) Commit() {
 	}
 	t.head = head
 	t.commitq = nil
+	return head
 }
 
 // Open opens new test transaction/connection.
@@ -473,13 +474,11 @@ func testPersistentDB(t0 *testing.T, rawcache bool) {
 
 	tdb.Add(101, "bonjour")
 	tdb.Add(102, "monde")
-	tdb.Commit()
-	at0 := tdb.head
+	at0 := tdb.Commit()
 
 	tdb.Add(101, "hello")
 	tdb.Add(102, "world")
-	tdb.Commit()
-	at1 := tdb.head
+	at1 := tdb.Commit()
 
 	tdb.Reopen() // so that at0 is not covered by db.δtail
 	db := tdb.db
@@ -541,8 +540,7 @@ func testPersistentDB(t0 *testing.T, rawcache bool) {
 
 	// commit change to obj2 from external process
 	tdb.Add(102, "kitty")
-	tdb.Commit()
-	at2 := tdb.head
+	at2 := tdb.Commit()
 
 	// new db connection should see the change
 	t2 := tdb.Open(&ConnOptions{})
@@ -730,8 +728,7 @@ func TestLiveCache(t0 *testing.T) {
 	tdb.Add(102, "труд")
 	tdb.Add(103, "май")
 	tdb.Add(104, "весна")
-	tdb.Commit()
-	at1 := tdb.head
+	at1 := tdb.Commit()
 
 	zcc := &zcacheControl{map[Oid]PCachePolicy{
 		// obj1 - default (currently: don't pin and don't keep state)