mixclient: Fix test timeouts

Tests were timing out due to mixpool checks that require KEs to not be
received too early for their stated epochs.  To speed tests up, the
mixclient's internal epoch ticker is manually ticked by the testing code, but
this was causing test runs where KE messages were being rejected by mixpool,
with the tests never completing and eventually timing out.

Using the current time as the epoch whenever the internal epoch ticker is
manually ticked by tests resolves the issue and allows all tests to pass.

Timeouts spotted by @jholdstock.

Author: jrick

mixing/mixclient/client.go

@@ -643,7 +643,7 @@ func (c *Client) waitForEpoch(ctx context.Context) (time.Time, error) {
 		if !timer.Stop() {
 			<-timer.C
 		}
-		return epoch, nil
+		return time.Now(), nil
 	case <-timer.C:
 		return epoch, nil
 	}