EthernetTetheringTest: use wrapper to send test UDP packet

Using send{Download, Upload}PacketUdp() in test
{testTetherUdpV4*, testTetherClatUdp} helps reduce the redundant
codes.

Bug: 237369591
Test: atest EthernetTetheringTest
Change-Id: Id1676567b9a9885ce8f65ff86a682653910ea150
1 file changed