tchannel: Fix flaky ResponseWriter test (#1941)
tchannel: Fix flaky ResponseWriter test (#1941)

This updates the TChannel `ResponseWriter` test to use a map when
comparing instead of raw bytes. This ensures that when we are
comparing multiple headers we do not need to worry about determinism.

This also updates the test to use subtests. This file was likely
created before subtests were introduced to Go :O

No functionality has changed.
yarpc/yarpc-go,yarpc/yarpc-go