test(logging): add back DeleteLog test (#3114)

fixes #1654 

To de-flake:
- Instead of calling Logs() which is flakey, we write a new log entry then do a single deletion
- Also verified with BE that the quota for all logadmin api calls is 600/min and 1k/hour

It might still flake for a new reason since DeleteLogs is getting deprecated and may be inherently flakey. I will monitor it.

logging/logging_test.go

@@ -473,38 +473,27 @@ func TestPing(t *testing.T) {
 	}
 }
 
-func TestLogsAndDelete(t *testing.T) {
-	t.Skip("https://github.com/googleapis/google-cloud-go/issues/1654")
-
-	// This function tests both the Logs and DeleteLog methods. We only try to
-	// delete those logs that we can observe and that were generated by this
-	// test. This may not include the logs generated from the current test run,
-	// because the logging service is only eventually consistent. It's
-	// therefore possible that on some runs, this test will do nothing.
+func TestDeleteLog(t *testing.T) {
 	ctx := context.Background()
-	it := aclient.Logs(ctx)
-	nDeleted := 0
-	for {
-		logID, err := it.Next()
-		if err == iterator.Done {
-			break
-		}
-		if err != nil {
-			t.Fatal(err)
-		}
-		if strings.HasPrefix(logID, testLogIDPrefix) {
-			if err := aclient.DeleteLog(ctx, logID); err != nil {
-				// Ignore NotFound. Sometimes, amazingly, DeleteLog cannot find
-				// a log that is returned by Logs.
-				if status.Code(err) != codes.NotFound {
-					t.Fatalf("deleting %q: %v", logID, err)
-				}
-			} else {
-				nDeleted++
-			}
+	initLogs()
+	c, a := newClients(ctx, testProjectID)
+	defer c.Close()
+	defer a.Close()
+	lg := c.Logger(testLogID)
+
+	if err := lg.LogSync(ctx, logging.Entry{Payload: "hello"}); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := aclient.DeleteLog(ctx, testLogID); err != nil {
+		// Ignore NotFound. Sometimes, amazingly, DeleteLog cannot find
+		// a log that is returned by Logs.
+		if status.Code(err) != codes.NotFound {
+			t.Fatalf("deleting %q: %v", testLogID, err)
 		}
+	} else {
+		t.Logf("deleted log_id: %q", testLogID)
 	}
-	t.Logf("deleted %d logs", nDeleted)
 }
 
 func TestNonProjectParent(t *testing.T) {