Add debug trace for MdnsIPv6LinkLocalWithDefaultRoute

The test is flaky and not local reproducible. Add logs with DEBUG level
and set logging level to DEBUG in the test. This commit is for debugging
purpose, which should be reverted when root cause is found.

Bug: 247693272
Bug: 246688231
Test: atest
Change-Id: I7bee788381ff245ae321f4816b81f71087bbeec2