Test: decouple the tests which access class member variables directly

This is a preparation for breaking down EthernetTetheringTest into
CTS and MTS tests.

EthernetTetheringTest is going to separate test base class and
derived class. In order to encapsulate base class better, decouple
the following tests which access member variables directly.
- testStaticIpv4
- testLocalOnlyTethering
- testPhysicalEthernet
- testVirtualEthernet
- testVirtualEthernetAlreadyExists

Above tests change to use local veriables instead of class member
variables: mDownstreamReader, mDownstreamIface,
mTetheringEventCallback, mTetheredInterfaceRequester.

This can help to decouple the tests with class member variables and
these class member variables can be private in the
base class EthernetTetheringTestBase since the following commit.

Bug: 250552545
Bug: 258637850
Test atest EthernetTetheringTest

Change-Id: Ifd855571337b34c1dfee5cb2791369de754bc94d
1 file changed