init() handed the logger &config.DefaultConfig.Logging, so the leveler's
mutex lived inside the global that NewConfig yaml-marshals. Marshalling
reflectively copies the whole struct, mutex included, which raced the
component-level lookups pion makes from its own goroutines and failed
TestPreferMediaCodecForPublisher under -race.
The values are the same; only the storage is now test-local. The integration
helper aliased the same global and gets the same treatment.