Some tests run with a xxx-master.opt file to set --thread_stack to a value
which is too small for mtr to actually bootstrap the server for testing.

Increase the minimum value for thread_stack for clang/ubsan, and suppress the
warnings generated by the server for the affected tests.

Change-Id: I7b93e1e9a922321aabcc9bbf4be0589e50c1c76f