Be less picky about TC filters in TetherOffloadForwarding test.

Instead of looking for strings that start with "filter protocol
ipv6 pref 2 bpf", use a regexp that checks for a BPF program
matching a reasonable regexp.

Bug: 176857251
Test: test-only change.
Change-Id: I98c5468a7bc2fe12e16381a6d835477299467737