Fix EtherentTetheringTest flaky

The first tethering test case may experience tethering restart
with IP conflict handling. Tethering would cache the last
upstreams so that the next enabled tethering avoids
picking up the address that is in conflict with the upstreams.
To protect subsequent tests, turn tethering on and off before
running them.

Bug: 254183718
Test: atest CtsTetheringTest
      atest MtsTetheringTestLatestSdk
Change-Id: I03729d3212a03bd9fcfe2c09cf327840f13b250c