Fix TestSendParallel by using only a single DB connection

see https://github.com/mattn/go-sqlite3/issues/274
see https://github.com/mattn/go-sqlite3/issues/209
see https://stackoverflow.com/questions/32479071/sqlite3-error-database-is-locked-in-golang

server/test/send_test.go

@@ -1429,8 +1429,6 @@ func TestQuotaExceededPro(t *testing.T) {
 }
 
 func TestSendParallel(t *testing.T) {
-	t.SkipNow()
-
 	_, baseUrl, stop := tt.StartSimpleWebserver(t)
 	defer stop()
 

server/test/util/webserver.go

@@ -61,6 +61,7 @@ func StartSimpleWebserver(t *testing.T) (*logic.Application, string, func()) {
 		RequestTimeout:    30 * time.Second,
 		ReturnRawErrors:   true,
 		DummyFirebase:     true,
+		DBSingleConn:      true,
 	}
 
 	sqlite, err := db.NewDatabase(conf)