shorten MultiTx test

The MultiTx test runs for a fairly long time but doesn't add much
value running that much longer, and there's no reason it should take
more than half the time we spend on this entire directory.

rbf/db_test.go

@@ -339,7 +339,7 @@ func TestDB_MultiTx(t *testing.T) {
 	}
 
 	// Continuously set/clear bits while readers are executing.
-	for i := 0; i < 1000; i++ {
+	for i := 0; i < 100; i++ {
 		func() {
 			tx, err := db.Begin(true)
 			if err != nil {