LB policy test framework: add helper methods and improve debuggability (grpc#31632)

* LB policy test framework: add helper methods and improve debuggability

* add ExpectPickFail() method

* fix build

* STL magic

* clang-tidy

* more clang-tidy

* add WaitForConnectionFailed() and log unexpected pick results

* add check for proper status for the connectivity state

Author: markdroth