bump timeouts on single-writer RBF Tx test

There's no correct timeout value here, really, but the intent
of this is that we first want to be sure that a second tx doesn't
successfully start before the first exits, and then that the second
*does* successfully start *after* the first exits.

Unfortunately, there's no guarantees on timely processing, and in
reality, CI can break us by waiting more than 10ms before we get
enough CPU time to do something. More generally, there's no way to
make a test like this work correctly -- no matter how long you wait
for the second Tx to start before closing the first one, it's always
possible that it *would* have started just a millisecond later even
without you closing the first one. And similarly, no matter how long
you give it to start when it's *supposed* to, it could always take
longer.

We could in principle just set this to wait for the second Tx to start
and rely on the test timeout killing us if it doesn't, but then we
don't get a useful message.

Let's optimistically hope that 10 seconds is long enough for a trivial
rollback to happen, since that doesn't need to imply writes. And I
think 50ms is a better bet for the first test, although that does
make this test close to 5x slower on non-CI hardware.

rbf/tx_test.go

@@ -140,14 +140,14 @@ func TestTx_CommitRollback(t *testing.T) {
 		select {
 		case <-ch1:
 			t.Fatal("second tx started while first tx active")
-		case <-time.After(10 * time.Millisecond):
+		case <-time.After(50 * time.Millisecond):
 		}
 
 		// Finish first transaction.
 		close(ch0)
 		select {
 		case <-ch1:
-		case <-time.After(10 * time.Millisecond):
+		case <-time.After(10 * time.Second):
 			t.Fatal("second tx should have started after first tx closed")
 		}
 	})