[EthernetTetheringTest] Don't run setupOnce() if Ethernet is not

supported.

BUG: 323896172
Test: atest CtsTetheringTest

Change-Id: Ibb01d9783a480f0c4acc9385a04527b95ae15fbb
Signed-off-by: Weilun Du <wdu@google.com>

Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java

@@ -176,6 +176,7 @@ public abstract class EthernetTetheringTestBase {
         // tests, turn tethering on and off before running them.
         MyTetheringEventCallback callback = null;
         TestNetworkInterface testIface = null;
+        assumeTrue(sEm != null);
         try {
             // If the physical ethernet interface is available, do nothing.
             if (isInterfaceForTetheringAvailable()) return;