Add missing assumeNoInterfaceForTetheringAvailable

Otherwise, if an ethernet network is connected via NCM during the test,
the request for a local-only NCM network will bring down global
connectivity and might break the test.

Test: TH
Flag: EXEMPT mainline
Change-Id: I5777b2a7658566348de7a346e0b394f10fdb0dd4

tests/cts/net/src/android/net/cts/EthernetManagerTest.kt

@@ -1236,6 +1236,8 @@ class EthernetManagerTest {
 
     @Test
     fun testNcmOnlyNetwork() {
+        assumeNoInterfaceForTetheringAvailable()
+
         setIncludeTestInterfaces(TEST_INTERFACE_MODE_NCM)
         val iface = createInterface()
 
@@ -1248,6 +1250,8 @@ class EthernetManagerTest {
 
     @Test
     fun testNonNcmNetwork() {
+        assumeNoInterfaceForTetheringAvailable()
+
         setIncludeTestInterfaces(TEST_INTERFACE_MODE_ETHERNET)
         val iface = createInterface()