EthernetTetheringTest: separate out tether IPv4 UDP BPF offload tests

Refactor test testTetherUdpV4* into basic tethering test and tethering
BPF offload tests which breakdown into three tests:
- testTetherUdpV4_VerifyBpf
- testKernelSupportBpfOffloadUdpV4
- testTetherConfigBpfOffloadEnabled

These tests provide a easy way to know if BPF offload is verified or
which condition caused a skip.

Example:
  Disable offload in tethering config
  packages/modules/Connectivity/Tethering/res/values/config.xml
  config_tether_enable_bpf_offload: false

  [..] testTetherUdpV4: PASSED (229ms)
  [..] testTetherUdpV4_VerifyBpf: ASSUMPTION_FAILED (2ms)
  [..] testKernelSupportBpfOffloadUdpV4: PASSED (1ms)
  [..] testTetherConfigBpfOffloadEnabled: ASSUMPTION_FAILED (13ms)

Bug: 237369591
Bug: 238288883

Test: atest EthernetTetheringTest
Change-Id: I99bd5c9d6be9a67cdeb809d46c907754f21ca5eb