[TestOnly]Create TetheringTester in initTetheringTester

If tethering's upstream have ipv6, the #initTetheringTester
would make sure ipv6 tethering connectivity is ready for testing.

Test: atest EthernentTetheringTest

Change-Id: Iade99eeb95fa94483248799eb852a81662bbb1e0
2 files changed